Nucleic acid molecules encoding alternansucrase

ABSTRACT

Nucleic acid molecules encoding an alternansucrase are provided. Moreover, vectors, host cells and plant cells transformed by the herein-described nucleic acid molecules and plants containing them are provided. Furthermore, methods are described for preparing transgenic plants which synthesize the carbohydrate alternan, because of the insertion of nucleic acid molecules encoding an alternansucrase. Moreover, methods for preparing alternan and products resulting from them are provided.

This application is a 37 C.F.R § 1.53(b) divisional of co-pending U.S. application Ser. No. 10/417,280 filed on Apr. 15, 2003, which is a divisional of application Ser. No. 09/499,203, filed on Feb. 8, 2000, now issued as U.S. Pat. No. 6,570,065, which claims priority of Application No. DE 19905069.4 filed in GERMANY on Feb. 8, 1999. The entire contents of each of the above-referenced applications are hereby incorporated by reference, and priority is claimed under 35 U.S.C. § 120.

The present invention relates to nucleic acid molecules encoding an alternansucrase. Moreover, this invention relates to vectors, host cells and plant cells transformed with the herein-described nucleic acid molecules, and plants containing said cells. Moreover, methods for preparing transgenic plants which due to the insertion of DNA molecules encoding an alternansucrase, synthesize the carbohydrate alternan, are described. Furthermore, methods for preparing alternan are described.

Prior art documents, the disclosure content of which is included into the present application by reference thereto, are cited hereinafter.

Alternan is a polysaccharide composed of glucose units. The glucose units are linked to each other via α-1,3- and α-1,6-glycosidic bonds, and said two types of bonds predominantly appear alternatingly. However, alternan is not a linear polysaccharide, but may contain branches (Seymour et al., Carbohydrate Research 74, (1979), 41-62). Because of its physico-chemical properties, the possibilities of application of alternan both in the pharmaceutical industry, for instance as a carrier of pharmaceutically active ingredients and as an additive in the textile, cosmetics and food industry have been discussed (Lopez-Munguia et al., Enzyme Microb. Technol. 15, (1993), 77-85; Leathers et al., Journal of Industrial Microbiology & Biotechnology 18, (1997), 278-283). Moreover, it can be used as a substitute for gum arabic (Coté, Carbohydrate Polymers 19, (1992), 249-252).

Industry has a high interest in biotechnological methods for preparing oligosaccharides and polysaccharides, and in particular alternan which is hardly or not at all accessible to classical organic synthesis. Compared to the classical approach of organic synthesis chemistry, biotechnological processes offer advantages. For instance, enzymatically catalyzed reactions as a rule show much higher specificities (regio specificity, stereo specificity) and higher reaction speeds, proceed under milder reaction conditions and lead to higher yields. These factors are of outstanding importance in the preparation of new oligosaccharides and polysaccharides.

Alternan is prepared enzymatically with the use of enzymes possessing the biological activity of alternansucrases. Alternansucrases belong to the group of glucosyltransferases, which, starting from saccharose, are able to catalyze the formation of alternan and fructose. So far, alternansucrases have only been found in the bacterium Streptococcus mutans (Mukasa et al. (J. Gen. Microbiol. 135 (1989), 2055-2063); Tsumori et al. (J. Gen. Microbiol. 131 (1985), 3347-3353)) and in specific strains of the gram positive bacterium Leuconostoc mesenteroides where they are, as a rule, present together with other polysaccharide-forming enzymes, such as for instance dextran-forming dextransucrases, or together with polysaccharide-degrading enzymes, such as alternanases. Hence, the naturally occurring strains also produce dextran in addition to alternan.

So far, alternan has been prepared in a cell-free system using partially purified proteins or by fermentation using alternansucrase-producing strains of Leuconostoc mesenteroides.

Various purification methods for the purification of alternansucrases have been described (Lopez-Munguia et al., Enzyme Microb. Technol. 15 (1993), 77-85; Lopez-Munguia et al., Annals New York Academy of Sciences 613 (1990), 717-722; Coté and Robyt, Carbohydrate Research 101 (1982), 57-74). These methods are complex and relatively costly, and, as a rule, lead to low protein yields (Leathers et al., Journal of Industrial Microbiology & Biotechnology 18 (1997), 278-283). None of these methods allows highly pure alternansucrase protein to be produced, and therefore sequencing of the protein and the isolation of the corresponding DNA sequences have not been successful so far. If the alternansucrase protein purified according to these methods is used for in vitro preparation of alternan, then the dextransucrase protein residues contained in the alternansucrase preparation produce dextran impurities in the alternan produced. The separation of alternan and dextran is relatively time-consuming and costly (Leathers et al., Journal of Industrial Microbiology & Biotechnology 18 (1997), 278-283). Another disadvantage of the dextransucrase protein impurities contained in the enzyme preparation of alternansucrase protein is the fact that a part of the saccharose substrate is converted into dextran and not into alternan, which results in a reduction of the alternan yield.

The fermentative preparation by means of Leuconostoc also leads to the formation of product mixtures of alternan and dextran. In order to increase the amount of alternansucrase from Leuconostoc strains, mutants have been isolated, such as the mutant NRRL B-21138, which secrete the alternansucrase and lead to a higher proportion of the amount of alternansucrase formed relative to dextransucrase. However, if such mutants are fermented with sucrose, the alternan obtained continues to show dextran impurities (Leathers et al., Journal of Industrial Microbiology & Biotechnology 18 (1997), 278-283).

As can be seen from the prior art discussed above, it has not been possible to provide highly purified alternansucrase protein so far.

Hence, the present invention addresses the problem of providing means and methods allowing alternan to be prepared in a time-saving and inexpensive manner.

This problem is solved by the provision of the embodiments characterized in the patent claims.

Consequently, the present invention relates to a nucleic acid molecule encoding a protein possessing the biological activity of an alternansucrase selected from the group consisting of

-   (a) nucleic acid molecules encoding at least the mature form of a     protein which comprises the amino acid sequence indicated in Seq. ID     No. 2 or the amino acid sequence encoded by the cDNA contained in     plasmid DSM 12666; -   (b) nucleic acid molecules comprising the nucleotide sequence     indicated in Seq. ID No. 1 or the nucleotide sequence of the cDNA     contained in plasmid DSM 12666 or a corresponding ribonucleotide     sequence; -   (c) nucleic acid molecules encoding a protein, the amino acid     sequence of which has a homology of at least 40% to the amino acid     sequence indicated in Seq. ID No. 2; -   (d) nucleic acid molecules, one strand of which hybridizes with the     nucleic acid molecules as defined in (a) or (b); -   (e) nucleic acid molecules comprising a nucleotide sequence encoding     a biologically active fragment of the protein which is encoded by     any one of the nucleic acid molecules as defined in (a), (b), (c) or     (d); and -   (f) nucleic acid molecules, the nucleotide sequence of which     deviates because of the degeneration of the genetic code from the     sequence of the nucleic acid molecules as defined in (a), (b),     (c), (d) or (e).

Consequently, the present invention relates to nucleic acid molecules encoding proteins possessing the biological activity of an alternansucrase, said molecules preferably encoding proteins comprising the amino acid sequence indicated in Seq. ID No. 2.

An enzyme possessing the enzymatic or biological activity of an alternansucrase (E.C. 2.4.1.140) is understood to mean an enzyme which is able to catalyze the conversion of saccharose into alternan and fructose. This conversion may occur both in the presence and absence of external acceptors (for instance maltose, isomaltose, isomaltotriose etc.). In the absence of external acceptors, alternansucrases starting from saccharose catalyze the release of fructose and high molecular alternan, a polysaccharide composed of glucose units, the backbone of which consists of glucose units predominantly connected to each other alternatingly by α-1,3- and α-1,6-glycosidic bonds. Concerning the percentage of α-1,3- and α-1,6-linked glucose units the literature displays different values. According to Mukasa et al. (J. Gen. Microbiol. 135 (1989), 2055-2063), alternan consists of 76 mol % α-1,3-linked glucose and 24 mol % α-1,6-linked glucose. Tsumori et al. (J. Gen. Microbiol. 131 (1985), 3347-3353) describe alternan as a polyglucan containing 49.1 mol % α-1,6-linked glucose and 33.9 mol % α-1,3-linked glucose with 13.6 mol % terminal glucose and 3.3 mol % α-1,3,6-branched glucose. In the presence of external acceptors, such as maltose, isomaltose, isomaltotriose and methyl-α-D-glucan, alternansucrase can catalyze the synthesis of α-D-glucan chains, in which the glucose residues are predominantly alternatingly connected by α-1,6- and α-1,3-glycosidic bonds, and the synthesis of fructose at these polysaccharide acceptors. Depending on the acceptor used, the products formed have different structures. The enzymatic activity of an alternansucrase can for instance be detected as described by Lopez-Munguia (Annals New York Academy of Sciences 613 (1990), 717-722) or as described in the examples of the present application.

The invention in particular relates to nucleic acid molecules containing the nucleotide sequence indicated under Seq. ID No. 1 or a part thereof, and preferably to molecules, which comprise the coding region indicated in Seq. ID No. 1 or corresponding ribonucleotide sequences.

Moreover, the present invention relates to nucleic acid molecules which encode an alternansucrase and the one strand of which hybridizes with one of the above-described molecules.

The present invention also relates to nucleic acid molecules which encode a protein, which has a homology, that is to say an identity of at least 40%, preferably at least 60%, preferably at least 70%, especially preferably at least 80% and in particular at least 90% to the entire amino acid sequence indicated in Seq. ID No. 2, the protein possessing the biological activity of an alternansucrase.

The present invention also relates to nucleic acid molecules, which encode an alternansucrase and the sequence of which deviates on account of the degeneration of the genetic code from the nucleotide sequences of the above-described nucleic acid molecules.

The invention also relates to nucleic acid molecules possessing a sequence which is complementary to the whole or a part of the above-mentioned sequences.

The nucleic acid sequence indicated in Seq. ID No. 1 for instance encodes an extracellular alternansucrase. Secretion is ensured by a signal sequence which comprises the first approximately 39 N-terminal amino acid groups of the Seq. ID No. 2. In certain circumstances it may be desirable for only the mature protein to be expressed without naturally occurring signal sequences and/or together with other signal sequences. Hence, the above-described nucleic acid molecules encode at least the mature form of a protein possessing the biological activity of an alternansucrase.

Within the present invention the term “hybridization” means hybridization under conventional hybridization conditions, preferably under stringent conditions, as for instance described in Sambrook et al., Molecular Cloning, A Laboratory Manual, 2^(nd) edition (1989) Cold Spring Harbour Laboratory Press, Cold Spring Harbour, N.Y. Within an especially preferred meaning the term “hybridization” means that hybridization occurs under the following conditions:

-   Hybridization buffer: 2×SSC; 10×Denhardt solution (Fikoll     400+PEG+BSA; ratio 1:1:1); 0.1% SDS; 5 mM EDTA;     -   50 mM Na₂HPO₄; 250 μg/ml of herring sperm DNA; 50 μg/ml of tRNA;         or     -   0.25 M of sodium phosphate buffer, pH 7.2;     -   1 mM EDTA     -   7% SDS -   Hybridization temperature T=60° C. -   Washing buffer: 2×SSC; 0.1% SDS -   Washing temperature T=60° C.

Nucleic acid molecules which hybridize with the nucleic acid molecules of the invention can, in principle, encode alternansucrases from any organism expressing such proteins.

Nucleic acid molecules which hybridize with the molecules of the invention can for instance be isolated from genomic libraries of microorganisms. Alternatively, they can be prepared by genetic engineering or chemical synthesis.

Such nucleic acid molecules may be identified and isolated with the use of the molecules of the invention or parts of these molecules or reverse complements of these molecules, for instance by hybridization according to standard methods (see for instance Sambrook et al., 1989, Molecular Cloning. A Laboratory Manual, 2^(nd) edition, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y.). Nucleic acid molecules possessing the same or substantially the same nucleotide sequence as indicated in Seq. ID No. 1 or parts thereof can, for instance, be used as hybridization probes. The fragments used as hybridization probes can also be synthetic fragments which are prepared by usual synthesis techniques, and the sequence of which substantially coincides with that of an inventive nucleic acid molecule.

The molecules hybridizing with the nucleic acid molecules of the invention also comprise fragments, derivatives and allelic variants of the above-described nucleic acid molecules encoding an alternansucrase of the invention. Herein, fragments are understood to mean parts of the nucleic acid molecules which are long enough to encode one of the described proteins, preferably showing the biological activity of an alternansucrase. In this connection, the term derivative means that the sequences of these molecules also differ from the sequences of the above-described nucleic acid molecules in one or more positions and show a high degree of homology to these sequences. In this context, homology means a sequence identity of at least 40%, in particular an identity of at least 60%, preferably more than 80% and particularly preferably more than 90%. Deviations from the above-described nucleic acid molecules may have been produced by deletion, substitution, insertion and/or recombination.

Preferably, the degree of homology is determined by comparing the respective sequence with the nucleotide sequence of the coding region of SEQ ID No.1. When the sequences which are compared do not have the same length, the degree of homology preferably refers to the percentage of nucleotide residues in the shorter sequence which is identical to nucleotide residues in the longer sequence. The degree of homology can be determined conventionally using known computer programs such as the ClustalW program (Thompson et al., Nucleic Acids Research 22 (1994), 4673-4680) distributed by Julie Thompson (Thompson@EMBL-Heidelberg.DE) and Toby Gibson (Gibson@EMBL-Heidelberg.DE) at the European Molecular Biology Laboratory, Meyerhofstrasse 1, D 69117 Heidelberg, Germany. ClustalW can also be downloaded from several websites including IGBMC (Institut de Genetique et de Biologie Moléculaire et Cellulaire, B.P.163, 67404 Illkirch Cedex, France; ftp://ftp-igbmc.u-strasbg.fr/pub/) and EBI (ftp://ftp.ebi.ac.uk/pub/software/) and all sites with mirrors to the EBI (European Bioinformatics Institute, Wellcome Trust Genome Campus, Hinxton, Cambridge CB10 1 SD, UK).

When using ClustalW program version 1.8 to determine whether a particular sequence is, for instance, 90% identical to a reference sequence according to the present invention, the settings are set in the following way for DNA sequence alignments:

KTUPLE=2, TOPDIAGS=4, PAIRGAP=5, DNAMATRIX: IUB, GAPOPEN=10, GAPEXT=5, MAXDIV=40, TRANSITIONS: unweighted.

For protein sequence alignments using ClustalW program version 1.8 the settings are the following: KTUPLE=1, TOPDIAG=5, WINDOW=5, PAIRGAP=3, GAPOPEN=10, GAPEXTEND=0.05, GAPDIST=8, MAXDIV=40, MATRIX=GONNET, ENDGAPS(OFF), NOPGAP, NOHGAP.

Furthermore, homology means preferably that the encoded protein displays a sequence identity of at least 40%, more preferably of at least 60%, even more preferably of at least 80%, in particular of at least 90% and particularly preferred of at least 95% to the amino acid sequence depicted under SEQ ID NO: 2. Homology, moreover, means that there is a functional and/or structural equivalence between the corresponding nucleic acid molecules or proteins encoded thereby. Nucleic acid molecules which are homologous to the above-described molecules and represent derivatives of these molecules are, as a rule, variations of these molecules which represent modifications having the same biological function. They may be either naturally occurring variations, for instance sequences from other microorganisms, or mutations, and said mutations may have formed naturally or may have been produced by deliberate mutagenesis. Furthermore, the variations may be synthetically produced sequences. The allelic variants may be naturally occurring variants or synthetically produced variants or variants produced by recombinant DNA techniques.

In a further preferred embodiment the term “derivative” encompasses a nucleic acid molecule coding for a protein which comprises at least one, more preferably at least three, even more preferably at least five, in particular at least ten and particularly preferred at least twenty of the peptide motifs selected from the group consisting of

a) MKQQE, (SEQ ID NO: 22) b) KKVPV, (SEQ ID NO: 23) c) KDDEN, (SEQ ID NO: 24) d) IDGNL, (SEQ ID NO: 25) e) YVADS, (SEQ ID NO: 26) f) HLRKN, (SEQ ID NO: 27) g) NENTP, (SEQ ID NO: 28) h) NVDGY, (SEQ ID NO: 29) i) NPDLK, (SEQ ID NO: 30) j) SNDSG, (SEQ ID NO: 31) k) NTFVK, (SEQ ID NO: 32) l) ISGYL, (SEQ ID NO: 33) m) SNAAL, (SEQ ID NO: 34) n) RQYTD, (SEQ ID NO: 35) o) QLYRA, (SEQ ID NO: 36) p) DDKAP, (SEQ ID NO: 37) q) TRQYT, (SEQ ID NO: 38) r) ITFAG, (SEQ ID NO: 39) s) NQYKG, (SEQ ID NO: 40) t) LFLNA, (SEQ ID NO: 41) u) QVSDT, (SEQ ID NO: 42) v) LITLN, (SEQ ID NO: 43) w) GRYVH, (SEQ ID NO: 44) x) TAPYG, (SEQ ID NO: 45) y) VVDYQ, (SEQ ID NO: 46) z) LSGQE. (SEQ ID NO: 47)

The proteins encoded by the different variants of the nucleic acid molecules of the invention possess certain characteristics they have in common. These include for instance enzymatic activity, molecular weight, immunological reactivity, conformation, etc., and physical properties, such as for instance the migration behavior in gel electrophoreses, chromatographic behavior, sedimentation coefficients, solubility, spectroscopic properties, stability, pH optimum, temperature optimum etc.

Alternansucrase (E.C. 2.4.1.140) is an enzyme belonging to the group of glucosyltransferases. So far, alternansucrase activity has not been found in plants, but only in the bacterium Streptococcus mutans (Mukasa et al. (J. Gen. Microbiol. 135 (1989), 2055-2063); Tsumori et al. (J. Gen. Microbiol. 131 (1985), 3347-3353)) and in specific strains of the bacterium Leuconostoc mesenteroides, for instance in NRRL B-1355, NRRL B-1498 and NRRL B-1501. As a rule, these strains contain different glucosyltransferases and secrete dextransucrases apart from alternansucrases if they are allowed to grow on saccharose-containing media. As a rule, these two sucrases possess a high binding affinity to the polysaccharides synthesized by them (Lopez-Munguia et al., Annals New York Academy of Sciences 613 (1990), 717-722) with the result that these polysaccharides must be separated from the protein in the purification of the enzymes from Leuconostoc mesenteroides strains grown on saccharose-containing medium (Lopez-Munguia et al., Enzyme Microb. Technol. 15 (1993), 77-85; Leathers et al., Journal of Industrial Microbiology & Biotechnology 18 (1997), 278-283).

In the absence of external acceptors, alternansucrases, starting from saccharose, catalyze the release of fructose and high molecular alternan, a polysaccharide which is composed of glucose units, and the backbone of which consists of glucose units predominantly linked to each other alternatingly by α-1,3- and α-1,6-glycosidic bonds and which according to light scattering measurement data should have a molecular weight of >107 (Coté, Carbohydrate Polymer 19 (1992), 249-252). To date there has been no report of alternan possessing a terminal fructose residue. Nevertheless, the existence of a terminal fructose unit in alternan can not be completely excluded. Lopez-Munguia et al. (Enzyme Microb. Technol. (1993) 77-85) describe that alternan is resistant to degradation by dextranases. However, it can be degraded by so-called alternanases, whereby ring-shaped oligomers of alternan of different polymerization degree can be produced (Biely et al., Eur. J. Biochem. 226 (1994), 633-639). Ultrasonic treatment of high molecular alternan allows the molecular weight of alternan to be reduced to <10⁶ (Coté, Carbohydrate Polymers 19 (1992), 249-252). If aqueous solutions of this ultrasonically treated alternan are prepared, then these solutions show rheological properties comparable to those of aqueous solutions of gum arabic. So-called “limit alternan” having a molecular weight of about 3500 can be produced by enzymatic degradation using isomaltodextranase from Arthrobacter globiformis (NRRL B-4425) (Coté, Carbohydrate Polymers 19 (1992), 249-252). In the presence of external acceptors, such as for instance maltose, isomaltose, isomaltotriose and methyl-α-D-glucan, alternansucrase catalyzes at said saccharide acceptors the synthesis of α-D-glucan chains, in which the glucose moieties are predominantly alternatingly linked by α-1,6- and α-1,3 glycosidic bonds, and the synthesis of fructose. Depending on the acceptor used, the resulting products have different structures and a molecular weight which is lower than that of high molecular alternan and a polymerization degree of <15. Because of the polymerization degree, these products are often also referred to as oligoalternans (Pelenc et al., Sciences Des Aliments 11 (1991), 465-476). However, within the framework of the present invention these low molecular products which can be prepared in the presence of external acceptors are also to be referred to as alternan.

In the preparation of oligoalternans by means of partially purified alternansucrase protein, maltose is an acceptor (Lopez-Munguia et al., Enzyme Microb. Technol. (1993), 77-85) producing high oligoalternan yields. Panose (degree of polymerization (d.p.) of 3) is the first acceptor product which is formed starting from maltose through the formation of an α-1,6-glycosidic bond. In contrast thereto, isomaltose is a less effective acceptor which leads to lower yields of oligoalternan (Lopez-Munguia et al., Enzyme Microb. Technol. 15 (1993), 77-85).

Alternansucrase is relatively stable and has a half life period of 2 days in 50 mM of acetate buffer, pH 5.4 at 40° C. (Lopez-Munguia et al., Enzyme Microb. Technol. (1993), 77-85). The enzyme shows maximum activity at a temperature of 40° C. and a pH value of 5.6 (Lopez-Munguia et al., Enzyme Microb. Technol. 15 (1993), 77-85).

In the absence of the substrate saccharose, alternansucrase catalyzes disproportionation reactions leading to a (partial) rearrangement of alternan. In particular when partially purified alternansucrase preparations containing dextransucrase contaminations were used to prepare oligoalternans, high disproportionation rates were found which lead to a complete rearrangement of oligoalternan (Lopez-Munguia et al., Enzyme Microb. Technol. 15 (1993), 77-85). For the molecular weight of alternansucrase according to SDS PAGE determination, different numerical values can be found: 135 kDa, 145 kDa, 173 kDa and 196 kDa, respectively (Leathers et al., Journal of Industrial Microbiology & Biotechnology 18 (1997), 278-283; Kim & Robyt, Enzyme Microb. Technol. 16 (1994), 659-664; Zhanley & Smith, Applied and Environmental Microbiology 61(3) (1995), 1120-1123).

The enzymatic activity of an alternansucrase can be shown for instance as described in Lopez-Munguia et al. (Annals New York Academy of Sciences 613 (1990), 717-722) or as described in the examples of the present application. One activity unit (1 u) can be defined as the amount of enzyme leading to the release of 1 μmol of fructose within one minute.

The nucleic acid molecules of the invention can be DNA molecules, in particular genomic molecules. Moreover, the nucleic acid molecules of the invention may be RNA molecules. The nucleic acid molecules of the invention can be obtained for instance from natural sources or may be produced synthetically or by recombinant techniques.

The nucleic acid molecules of the invention allow host cells to be prepared which produce recombinant alternansucrase protein of high purity and/or in sufficient quantities, and genetically engineered plants possessing an activity of these enzymes leading to the formation of alternan in planta. Within the framework of the present invention the term “high purity” means that the protein according to the invention displays a degree of purity of at least 80%, preferably of at least 90%, even more preferably of at least 95%. Moreover, means and methods are provided which may be used for preparing alternan using host cells and/or for preparing recombinant alternansucrase protein. Consequently, the provision of the nucleic acid molecules of the invention permits the preparation of alternan of high purity by methods which are relatively inexpensive and consume relatively little time.

In a preferred embodiment, the nucleic molecules of the invention are derived from microorganisms, preferably from bacteria, more preferably from gram-positive bacteria and in particular preferably from bacteria belonging to the genus Leuconostoc. Nucleic acid molecules from bacteria belonging to the species Leuconostoc mesenteroides are particularly preferred.

The invention also relates to oligonucleotides specifically hybridizing to a nucleic acid molecule of the invention. Such oligonucleotides have a length of preferably at least 10, in particular at least 15, and particularly preferably of at least 50 nucleotides. They are characterized in that they specifically hybridize to the nucleic acid molecules of the invention, that is to say that they do not or only to a very minor extent hybridize to nucleic acid sequences encoding other proteins, in particular other glucosyltransferases. The oligonucleotides of the invention can be used for instance as primers for amplification techniques such as the PCR reaction or as a hybridization probe to isolate related genes.

Moreover, the invention relates to vectors, in particular plasmids, cosmids, viruses, bacteriophages and other vectors commonly used in gene technology, which contain the above-described nucleic acid molecules of the invention. In a preferred embodiment of the invention, the vectors of the invention lend themselves to the transformation of fungal cells or cells of microorganisms. Preferably, such vectors are suitable to transform plant cells. Particularly preferably, such vectors permit the integration of the nucleic acid molecules of the invention, possibly together with flanking regulatory regions, into the genome of the plant cell. Examples thereof are binary vectors which can be used in the Agrobacteria-mediated gene transfer, and some are already commercially available.

In another preferred embodiment, the nucleic acid molecules contained in the vectors are connected to regulatory elements ensuring the transcription and synthesis of a translatable RNA in prokaryotic or eukaryotic cells.

The expression of the nucleic acid molecules of the invention in prokaryotic or eukaryotic cells, for instance in Escherichia coli, is interesting because it permits a more precise characterization of the enzymatic activities of the enzymes encoded by these molecules. Moreover, it is possible to express these enzymes in such prokaryotic or eukaryotic cells which are free from interfering enzymes, such as dextransucrases or other polysaccharide-forming or polysaccharide-degrading enzymes. In addition, it is possible to insert different mutations into the nucleic acid molecules by methods usual in molecular biology (see for instance Sambrook et al., 1989, Molecular Cloning, A Laboratory Manual, 2^(nd) edition, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y.), leading to the synthesis of proteins possibly having modified biological properties. On the one hand it is possible in this connection to produce deletion mutants in which nucleic acid molecules are produced by progressive deletions from the 5′ or 3′ end of the coding DNA sequence, and said nucleic acid molecules lead to the synthesis of correspondingly shortened proteins. Such deletions at the 5′ end of the nucleotide sequence for instance allow amino acid sequences to be identified which are responsible for the secretion of the enzyme in microorganisms (transit peptides). This permits the deliberate preparation of enzymes which are no longer secreted by the removal of the corresponding sequences, but remain within the cell of the corresponding host organism or are localized in other compartments, for instance in the plastids, mitochondria, vacuole, on account of the addition of other signal sequences.

On the other hand, the introduction of point mutations is also conceivable at positions at which a modification of the amino acid sequence for instance influences the enzyme activity or the control of the enzyme. In this manner, it is for instance possible to produce mutants which possess a modified stereo and regio selectivity or a modified K_(m) value or which are no longer subject to the control mechanisms normally existing in the cell and realized via an allosteric control or covalent modification.

Moreover, mutants possessing a modified substrate or product specificity can be prepared. Furthermore, it is possible to prepare mutants having a modified activity-temperature-profile.

Furthermore, in the case of expression in plants, the insertion of mutations into the nucleic acid molecules of the invention allows the gene expression rate and/or the activity of the proteins encoded by the nucleic acid molecules of the invention to be increased.

For genetic engineering in prokaryotic cells, the nucleic acid molecules of the invention or parts of these molecules can be introduced into plasmids which permit mutagenesis or sequence modification by recombination of DNA sequences. Standard methods (see Sambrook et al., 1989, Molecular Cloning: A laboratory manual, 2^(nd) edition, Cold Spring Harbor Laboratory Press, NY, USA) allow base exchanges to be performed or natural or synthetic sequences to be added. DNA fragments can be connected to each other by applying adapters and linkers to the fragments. Moreover, engineering measures which provide suitable restriction sites or remove surplus DNA or restriction sites can be used. In those cases, in which insertions, deletions or substitutions are possible, in vitro mutagenesis, “primer repair”, restriction or ligation can be used. In general, a sequence analysis, restriction analysis and other methods of biochemistry and molecular biology are carried out as analysis methods.

Moreover, the invention relates to plasmid pAlsu-pSK (see FIG. 2 and Example 2) which was deposited at Deutsche Sammlung für Mikroorganismen und Zellkulturen (DSMZ), Braunschweig, under the accession No. DSM 12666 on Feb. 4, 1999, and to the nucleic acid molecules contained in the insert of plasmid DSM 12666 and encoding a protein possessing the enzymatic activity of an alternansucrase. Moreover, the present invention also relates to nucleic acid molecules which hybridize to the insertion of plasmid DSM 12666. Also, the present invention relates to nucleic acid molecules the nucleotide sequence of which deviates from that of the nucleic acid molecules of the plasmid DSM 12666 insert, because of the degeneration of the genetic code. Furthermore, the present invention relates to nucleic acid molecules which have a homology, that is to say a sequence identity of at least 40%, preferably of at least 60%, more preferably of at least 80%, even more preferably of at least 90%, and most preferably of at least 95% to the sequence of the insertion of plasmid DSM 12666.

Another embodiment of the invention relates to host cells, in particular prokaryotic or eukaryotic cells transformed with an above-described nucleic acid molecule of the invention or with a vector of the invention, and to cells descended from such transformed cells and containing a nucleic acid molecule or vector of the invention.

According to another preferred embodiment, the host cells are cells of microorganisms. In the context of the present invention, the term “microorganism” comprises bacteria and all protists (e.g. fungi, in particular yeasts, algae) as defined Schlegel's “Allgemeine Mikrobiologie” (Georg Thieme Verlag, 1985, 1-2). A preferred embodiment of the invention relates to cells of algae and host cells belonging to the genera Aspergillus, Bacillus, Saccharomyces or Pichia (Rodriguez, Journal of Biotechnology 33 (1994), 135-146, Romanos, Vaccine, Vol. 9 (1991), 901 et seq.). A particularly preferred embodiment of the invention relates to E. Coli cells. Alternansucrase is especially preferably secreted by the host cell. The preparation of such host cells for the production of recombinant alternansucrase can be carried out by methods known to a man skilled in the art. In a preferred embodiment of the invention, the host cells of the invention show no interfering enzymatic activities, such as those of polysaccharide-forming and/or polysaccharide-degrading enzymes.

An overview of different expression systems is for instance contained in Methods in Enzymology 153 (1987), 385-516, in Bitter et al. (Methods in Enzymology 153 (1987), 516-544) and in Sawers et al. (Applied Microbiology and Biotechnology 46 (1996), 1-9), Billman-Jacobe (Current Opinion in Biotechnology 7 (1996), 500-4), Hockney (Trends in Biotechnology 12 (1994), 456-463), Griffiths et al, Methods in Molecular Biology 75 (1997), 427-440). An overview of yeast expression systems is for instance given by Hensing et al. (Antonie van Leuwenhoek 67 (1995), 261-279), Bussineau et al. (Developments in Biological Standardization 83 (1994), 13-19), Gellissen et al. (Antonie van Leuwenhoek 62 (1992), 79-93, Fleer (Current Opinion in Biotechnology 3 (1992), 486-496), Vedvick (Current Opinion in Biotechnology 2 (1991), 742-745) and Buckholz (Bio/Technology 9 (1991), 1067-1072).

Expression vectors have been widely described in the literature. As a rule, they contain not only a selection marker gene and a replication-origin ensuring replication in the host selected, but also a bacterial or viral promoter, and in most cases a termination signal for transcription. Between the promoter and the termination signal there is at least one restriction site or a polylinker which enables the insertion of a coding DNA sequence. The DNA sequence naturally controlling the transcription of the corresponding gene can be used as the promoter sequence, if it is active in the selected host organism. However, this sequence can also be exchanged for other promoter sequences. It is possible to use promoters producing a constitutive expression of the gene and inducible promoters which permit a deliberate control of the expression of the postconnected gene. Bacterial and viral promoter sequences possessing these properties are described in detail in the literature. Regulatory sequences for the expression in microorganisms (for instance E. coli, S. cerevisiae) are sufficiently described in the literature. Promoters permitting a particularly high expression of the postconnected gene are for instance the T7 promoter (Studier et al., Methods in Enzymology 185 (1990), 60-89), lacUV5, trp, trp-lacUV5 (DeBoer et al., in Rodriguez and Chamberlin (Eds), Promoters, Structure and Function; Praeger, New York, (1982), 462-481; DeBoer et al., Proc. Natl. Acad. Sci. USA (1983), 21-25), Ip1, rac (Boros et al., Gene 42 (1986), 97-100). As a rule, the protein amounts are highest from the middle up to about the end of the logarithmic phase of the growth cycle of the microorganisms. Therefore, inducible promoters are preferably used for the synthesis of proteins. These promoters often lead to higher protein yields than do constitutive promoters. The use of highly constitutive promoters leads to the continuous transcription and translation of a cloned gene and thus often has the result that energy is lost for other essential cells functions with the effect that cell growth is slowed down (Bernard R. Glick/Jack J. Pasternak, Molekulare Biotechnologie (1995). Spektrum Akademischer Verlag GmbH, Heidelberg, Berlin, Oxford, p. 342). Therefore, in order to obtain an optimum amount of protein, a two-stage process is often used. First, the host cells are cultured under optimum conditions up to a relatively high cell density. In the second step, transcription is then induced depending on the type of promoter used. In this connection, a tac promoter is particularly suitable which can be induced by lactose or IPTG (=isopropyl-β-D-thiogalactopyranoside) (deBoer et al., Proc. Natl. Acad. Sci. USA 80 (1983), 21-25). Termination signals for transcription are also described in the literature.

The transformation of the host cell with DNA encoding an alternansucrase can, as a rule, be carried out by standard methods, as for instance described in Sambrook et al., (Molecular Cloning: A Laboratory Course Manual, 2^(nd) edition (1989) Cold Spring Harbor Press, New York; Methods in Yeast Genetics, A Laboratory Course Manual, Cold Spring Harbor Laboratory Press, 1990). The host cell is cultured in nutrient media meeting the requirements of the particular host cell used, in particular in respect of the pH value, temperature, salt concentration, aeration, antibiotics, vitamins, trace elements etc.

Moreover, the invention relates to proteins and biologically active fragments thereof, which are encoded by the nucleic acid molecules of the invention and to methods for their preparation, wherein a host cell according to the invention is cultured under conditions permitting the synthesis of the protein, and the protein is subsequently isolated from the cultured cells and/or the culture medium.

According to a preferred embodiment of the invention, the alternansucrase is a recombinantly produced protein. In the context of the present invention, this is a protein prepared by inserting a DNA sequence encoding the protein into a host cell and expressing it therein. The protein can then be isolated from the host cell and/or the culture medium.

The nucleic acid molecules of the invention now allow host cells to be prepared which produce recombinant alternansucrase protein of high purity and/or in sufficient amounts. Within the framework of the present invention the term “high purity” means that the protein according to the invention displays a degree of purity of at least 80%, preferably of at least 90%, even more preferably of at least 95%. The time-consuming and costly methods already mentioned above, whereby alternansucrase protein which to date can only be obtained from particular Leuconostoc strains can be purified from other components such as for instance dextransucrases, polysaccharides, are dispensed with, because alternansucrase can be produced in host cells not possessing any adverse polysaccharide-synthesizing activities. Moreover, host cells and vectors can also be used, which allow the alternansucrase protein to be produced in the absence of saccharose, with the result that an additional separation of the alternansucrase protein from polysaccharides is no longer necessary. Moreover, the selection of suitable host cells and vectors allows alternansucrase protein to be provided in sufficient amounts, which has not been possible with the systems so far described. Alternansucrase produced by the host cells can be purified by conventional purification methods, such as precipitation, ion exchange chromatography, affinity-chromatography, gel filtration, HPLC Reverse Phase Chromatography etc. The modification of the nucleic acid molecules of the invention encoding an alternansucrase and expressed in the host cells, allow to produce a polypeptide in the host cell which is easier to isolate from the culture medium because of particular properties. Thus, the protein to be expressed can be expressed as a fusion protein with an additional polypeptide sequence, the specific binding properties of which permit the isolation of the fusion protein by affinity chromatography (e.g. Hopp et al., Bio/Technology 6 (1988), 1204-1210; Sassenfeld, Trends Biotechnol. 8 (1990), 88-93).

Another embodiment of the invention relates to proteins possessing the enzymatic activity of an alternansucrase, in particular that from microorganisms, preferably Gram-positive microorganisms, particularly microorganisms of the genus Leuconostoc, and particularly preferably that from Leuconostoc mesenteroides. The molecular weight of the protein indicated in Seq. ID No. 2, as determined by calculation, is 228.96 kDa. The invention also relates to alternansucrases which possess a molecular weight of 229 kDa±120 kDa, preferably 229 kDa±50 kDa, and particularly preferably 230 kDa±25 kDa. The molecular weight of the mature protein, as determined by calculation, is 224.77 kDa.

The provision of the nucleic acid molecules of the invention, for the first time, makes it possible to prepare alternansucrase-expressing plant cells by means of genetic engineering, which was not possible so far, because classical culturing methods do not allow bacterial and fungal genes to be expressed in plants. The invention, therefore, also relates to transgenic plant cells transformed by a nucleic acid molecule of the invention or a vector of the invention or descended from such cells, the nucleic acid molecule which encodes the protein that has the biological activity of an alternansucrase being under the control of regulatory elements permitting the transcription of a translatable mRNA in plant cells. The introduction of the activity of the proteins of the invention, for instance by expression of corresponding nucleic acid molecules, opens the possibility of producing alternan in plant cells correspondingly modified by genetic engineering. Hence, the expression of the nucleic acid molecules of the invention in plant cells is possible, allowing an additional, corresponding alternansucrase activity not present in the wild type to be introduced. Moreover, it is possible to modify the nucleic acid molecules of the invention according to methods known to a skilled person, in order to obtain alternansucrases of the invention which for instance possess modified temperature dependencies or substrate or product specificities. Such methods have already been described in more detail in a different context above.

A plurality of techniques is available by which DNA can be inserted into a plant host cell. These techniques include the transformation of plant cells by T-DNA using Agrobacterium tumefaciens or Agrobacterium rhizogenes as a transforming agent, the fusion of protoplasts, injection, electroporation of DNA, insertion of DNA by the biolistic approach and other possibilities.

The use of the Agrobacteria-mediated transformation of plant cells has been extensively investigated and sufficiently described in EP 120 516; Hoekema, In: The Binary Plant Vector System, Offsetdrukkerij Kanters B. V., Alblasserdam (1985), Chapter V; Fraley et al, Crit. Rev. Plant Sci. 4 (1993), 1-46 and An et al., EMBO J. 4 (1985), 277-287. Regarding the transformation of potatoes see for instance Rocha-Sosa et al. (EMBO J. 8 (1989), 29-33).

The transformation of monocotyledonous plants by means of Agrobacterium-based vectors has also been described (Chan et al., Plant Mol. Biol. 22 (1993), 491-506; Hiei et al., Plant J. 6 (1994) 271-282; Deng et al, Science in China 33 (1990), 28-34; Wilmink et al, Plant Cell Reports 11 (1992), 76-80; May et al., Bio/Technology 13 (1995), 486-492; Conner and Dormisse, Int. J. Plant Sci. 153 (1992), 550-555; Ritchie et al. Transgenic Res. 2 (1993), 252-265). An alternative system for transforming monocotyledonous plants is the transformation by the biolistic approach (Wan and Lemaux, Plant Physiol. 104 (1994), 37-48; Vasil et al., Bio/Technology 11 (1993), 1553-1558; Ritala et al., Plant Mol. Biol. 24 (1994) 317-325; Spencer et al., Theor. Appl. Genet. 79 (1990), 625-631), protoplast transformation, electroporation of partially permeabilized cells, insertion of DNA via glass fibers. The transformation of maize in particular has been repeatedly described in the literature (see for instance WO 95/06128, EP 0 513 849, EP 0 465 875, EP 29 24 35; Fromm et al, Biotechnology 8, (1990), 833-844; Gordon-Kamm et al., Plant Cell 2, (1990), 603-618; Koziel et al., Biotechnology 11 (1993), 194-200; Moroc et al., Theor. Appl. Genet. 80, (1990), 721-726).

The successful transformation of other types of cereals has also been described for instance of barley (Wan and Lemaux, supra; Ritala et al., supra, Krens et al., Nature 296 (1982), 72-74) and wheat (Nehra et al., Plant J. 5 (1994), 285-297). Generally, any promoter active in plant cells is suitable to express the nucleic acid molecules in plant cells. The promoter can be so chosen that the expression in the plants of the invention occurs constitutively or only in a particular tissue, at a particular time of plant development or at a time determined by external influences. The promoter may be homologous or heterologous to the plant. Suitable promoters are for instance the promoter of 35S RNA of the Cauliflower Mosaic Virus (see for instance U.S. Pat. No. 5,352,605) and the ubiquitin-promoter (see for instance U.S. Pat. No. 5,614,399) which lend themselves to constitutive expression, the patatin gene promoter B33 (Rocha-Sosa et al., EMBO J. 8 (1989), 23-29) which lends itself to a tuber-specific expression in potatoes or a promoter ensuring expression in photosynthetically active tissues only, for instance the ST-LS1 promoter (Stockhaus et al., Proc. Natl. Acad. Sci. USA 84 (1987), 7943-7947; Stockhaus et al., EMBO, J. 8 (1989) 2445-2451), the Ca/b-promoter (see for instance U.S. Pat. No. 5,656,496, U.S. Pat. No. 5,639,952, Bansal et al., Proc. Natl. Acad. Sci. USA 89 (1992), 3654-3658) and the Rubisco SSU promoter (see for instance U.S. Pat. No. 5,034,322; U.S. Pat. No. 4,962,028) or the glutelin promoter from wheat which lends itself to endosperm-specific expression (HMW promoter) (Anderson, Theoretical and Applied Genetics 96, (1998), 568-576, Thomas, Plant Cell 2 (12), (1990), 1171-1180), the glutelin promoter from rice (Takaiwa, Plant Mol. Biol. 30(6) (1996), 1207-1221, Yoshihara, FEBS Lett. 383 (1996), 213-218, Yoshihara, Plant and Cell Physiology 37 (1996), 107-111), the shrunken promoter from maize (Maas, EMBO J. 8 (11) (1990), 3447-3452, Werr, Mol. Gen. Genet. 202(3) (1986), 471-475, Werr, Mol. Gen. Genet. 212(2), (1988), 342-350), the USP promoter, the phaseolin promoter (Sengupta-Gopalan, Proc. Natl. Acad. Sci. USA 82 (1985), 3320-3324, Bustos, Plant Cell 1 (9) (1989), 839-853) or promoters of zein genes from maize (Pedersen et al., Cell 29 (1982), 1015-1026; Quatroccio et al., Plant Mol. Biol. 15 (1990), 81-93). However, promoters which are only activated at a point in time determined by external influences can also be used (see for instance WO 93/07279). In this connection, promoters of heat shock proteins which permit simple induction may be of particular interest. Moreover, seed-specific promoters such as the USP promoter from Vicia faba which ensures a seed-specific expression in Vicia faba and other plants may be used (Fiedler et al., Plant Mol. Biol. 22 (1993), 669-679; Bäumlein et al., Mol. Gen. Genet. 225 (1991), 459-467). Moreover, fruit-specific promoters, such as described in WO 91/01373 may be used too.

Moreover, a termination sequence may be present, which serves to terminate transcription correctly and to add a poly-A-tail to the transcript, which is believed to have a function in the stabilization of the transcripts. Such elements are described in the literature (see for instance Gielen et al., EMBO J. 8 (1989), 23-29) and can be replaced at will.

Such cells can be distinguished from naturally occurring plant cells inter alia by the fact that they contain a nucleic acid molecule of the invention which does not naturally occur in these cells. Moreover, such transgenic plant cells of the invention can be distinguished from naturally occurring plant cells in that they contain at least one copy of the nucleic acid molecule of the invention stably integrated in their genome.

Moreover, the plant cells of the invention can preferably be distinguished from naturally occurring plant cells by at least one of the following features: If the inserted nucleic acid molecule of the invention is heterologous to the plant cell, then the transgenic plant cells are found to have transcripts of the inserted nucleic acid molecules of the invention. The latter can be detected for instance by Northern blot analysis. The plants cells of the invention preferably contain a protein encoded by an inserted nucleic acid molecule of the invention. This can be shown for instance by immunological methods, in particular by Western blot analysis.

Transgenic plant cells can be regenerated to whole plants according to methods known to a person skilled in the art.

The present invention also relates to the plants obtainable by regeneration of the transgenic plant cells of the invention. Furthermore, it relates to plants containing the above-described transgenic plant cells.

In most plants, the photoassimilates in the form of sugars formed during photosynthesis within a plant, i.e. mainly in the form of saccharose, are transported to the corresponding target organs. As saccharose is the substrate of the polymerization reaction of alternansucrase, all plants, both monocotyledonous and dicotyledonous can, in principle, be modified by the nucleic acid molecule of the invention in respect of alternansucrase expression.

The expression in plants of the nucleic acid molecules of the invention encoding a protein having the enzymatic activity of an alternansucrase can, for instance, be used to achieve a modification of the viscosity of the extracts possibly obtained from the plants, said modification being achieved by the synthesis of alternan. In this connection, for instance tomatoes are of interest. The expression of an alternansucrase in a tomato fruit leads to the synthesis of alternan and results in a modification of the viscosity of extracts obtained from these fruits for instance for the production of tomato puree or tomato ketchup.

The expression of the nucleic acid molecules of the invention is in particular advantageous in those organs of the plant which show a higher saccharose content or store saccharose. Such organs are for instance the beet of sugar beet or the cane of sugar cane. As these plants normally do not store any appreciable amounts of starch, the alternans synthesized by the alternansucrase from these plants could be isolated in the pure form.

The site where the biosynthesis of the saccharose in the plant cell occurs is the cytosol. The storage site, however, is the vacuole. During its transport into the storage tissue of the sugar beet or the potato or during its transport into the endosperm of seeds, the saccharose must pass the apoplast. Hence, all three compartments, i.e. the cytosol, the vacuole, the apoplast, lend themselves to the expression of the nucleic acid molecules for the synthesis of alternan. In addition, the plastids also lend themselves thereto, as could for instance be shown by the expression of bacterial fructosyl transferases in amyloplasts. Said fructosyl transferases which likewise require saccharose as a substrate, were able to mediate the formation of “amylofructan” in amyloplasts (Smeekens, Trends in Plant Science, Vol. 2, No. 8 (1997), 286-288).

In the case of starch-producing plants, such as potatoes and maize, where the starch biosynthesis and starch storage normally take place in the amyloplasts, an expression of the alternansucrase in apoplasts, in the cytosol or in the vacuole would lead to an additional synthesis of oligosaccharides and/or polysaccharides in these compartments, which can mean an overall increase in the yield.

As in the case of potatoes the starch synthesized in the amyloplasts can be separated from the alternan synthesized in the apoplast, in the cytosol or in the vacuole, the very same plant can be used to recover starch and alternan. Moreover, transgenic potato and maize plants are known, the starch synthesis of which in the tubers and grains, respectively, is completely inhibited due to the inhibition of ADP-glucose-pyrophosphorylase by an antisense construct. In the case of potatoes, soluble sugars, in particular saccharose and glucose, accumulate instead, for instance in the tubers (Müller-Röber et al., EMBO J. 11 (1992), 1229-1238). Alternan can be prepared in the cytosol, the vacuole or apoplast of these plants by the expression of an alternansucrase which uses saccharose as a substrate.

Therefore in another embodiment of the invention the plant cells of the invention are further characterized by a reduced ADP glucose pyrophosphorylase (AGPase) activity compared to corresponding cells from wild-type plants.

DNA molecules encoding AGPase are well known to the person skilled in the art and described for example in Müller-Röber et al. (Mol. Gen. Genet. 224 (1) (1990), 136-146). By using DNA molecules encoding an AGPase it is possible to produce plants by means of recombinant DNA techniques (for example by an antisense, a ribozyme or a cosuppression approach) showing a reduced AGPase activity. Furthermore AGPase mutants, for example from maize (brittle-2 and shrunken-2), with reduced AGPase activity are known to the person skilled in the art.

The term “reduced” means preferably a reduction of AGPase activity of at least 10%, more preferably of at least 50% and even more preferably of at least 80% in comparison to corresponding wild-type cells.

The activity of an AGPase can be determined according to Müller-Röber et al. (Mol. Gen. Genet. 224 (1) (1990), 136-146) or to methods known to a person skilled in the art.

The reaction which is catalyzed by alternansucrase is distinguished by the fact that a glucose moiety is transferred directly from saccharose to an existing carbohydrate acceptor. By contrast, in the case of plants, the biosynthesis of linear glucans from saccharose, proceeds in such a way that the saccharose is first separated into glucose and fructose, which are then each converted into activated intermediate ADP-glucose. The glucose moiety is transferred by the enzyme starch-synthase from the ADP glucose to an already existing glucan, whereby ADP is released. The conversion of saccharose into two ADP glucose molecules requires several energy consuming reactions. Therefore, the energy consumption of the reaction catalyzed by alternansucrase is substantially lower than the energy consumption in the synthesis of polysaccharides from saccharose in plant cells, which can lead to an increased yield of synthesized oligo and/or polysaccharides in plants containing the nucleic acid molecules of the invention.

In the expression of the nucleic acid molecules in plants there exists in principle the possibility that the synthesized protein can be localized in any compartment of the plant cell (e.g. in the cytosol, plastids, vacuole, mitochondria) or the plant (e.g. in the apoplast). In order to achieve the localization in a particular compartment, the coding region must, where necessary, be linked to DNA sequences ensuring localization in the corresponding compartment. The signal sequences used must each be arranged in the same reading frame as the DNA sequence encoding the enzyme.

In order to ensure the location in the plastids it is conceivable to use one of the following transit peptides: of the plastidic Ferredoxin: NADP₊ oxidoreductase (FNR) of spinach which is enclosed in Jansen et al. (Current Genetics 13 (1988), 57-522). In particular, the sequence ranging from the nucleotides—171 to 165 of the cDNA Sequence disclosed therein can be used, which comprises the 5′non-translated region as well as the sequence encoding the transit peptide. Another example is the transit peptide of the waxy protein of maize including the first 34 amino acid residues of the mature waxy protein (Klösgen et al., Mol. Gen. Genet. 217 (1989), 155-161). It is also possible to use this transit peptide without the first 34 amino acids of the mature protein. Furthermore, the signal peptides of the ribulose bisposphate carboxylase small subunit (Wolter et al., Proc. Natl. Acad. Sci. USA 85 (1988), 846-850; Nawrath et al., Proc. Natl. Acad. Sci. USA 91 (1994), 12760-12764), of the NADP malat dehydrogenase (Gallardo et al., Planta 197 (1995), 324-332), of the glutathione reductase (Creissen et al., Plant J. 8 (1995), 167-175) or of the R1 protein Lorberth et al. (Nature Biotechnology 16, (1998), 473-477) can be used.

In order to ensure the location in the vacuole it is conceivable to use one of the following transit peptides: the N-terminal sequence (146 amino acids) of the patatin protein (Sonnewald et al., Plant J. 1 (1991), 95-106) or the signal sequences described by Matsuoka und Neuhaus, Journal of Experimental Botany 50 (1999), 165-174; Chrispeels und Raikhel, Cell 68 (1992), 613-616; Matsuoka und Nakamura, Proc. Natl. Acad. Sci. USA 88 (1991), 834-838; Bednarek und Raikhel, Plant Cell 3 (1991), 1195-1206; Nakamura und Matsuoka, Plant Phys. 101 (1993), 1-5.

In order to ensure the location in the mitochondria it is for example conceivable to use the transit peptide described by Braun et al.(EMBO J. 11, (1992), 3219-3227).

In order to ensure the location in the apoplast it is conceivable to use one of the following transit peptides: signal sequence of the proteinase inhibitor 11-gene (Keil et al., Nucleic Acid Res. 14 (1986), 5641-5650; von Schaewen et al., EMBO J. 9 (1990), 30-33), of the levansucrase gene from Erwinia amylovora (Geier and Geider, Phys. Mol. Plant. Pathol. 42 (1993), 387-404), of a fragment of the patatin gene B33 from Solanum tuberosum, which encodes the first 33 amino acids (Rosahl et al., Mol. Gen. Genet. 203 (1986), 214-220) or of the one described by Oshima et al. (Nucleic Acid Res. 18 (1990), 181).

The nucleic acid sequence indicated in Seq. ID No. 1 encodes an extracellular alternansucrase. Secretion is ensured by a signal sequence comprising the first approximately 39 N-terminal amino acid residues of the Seq. ID No. 2.

The transgenic plants may, in principle, be plants of any plant species, that is to say they may be monocotyledonous and dicotyledonous plants. Preferably, the plants are useful plants cultivated by man for nutrition or for technical, in particular industrial, purposes. They are preferably starch-storing plants, for instance cereal species (rye, barley, oat, wheat, millet, sago etc.), rice, pea, marrow pea, cassaya and potato; tomato, rape, soybean, hemp, flax, sunflower, cow pea or arrowroot, fiber-forming plants (e.g. flax, hemp, cotton), oil-storing plants (e.g. rape, sunflower, soybean) and protein-storing plants (e.g. legumes, cereals, soybeans). The invention also relates to fruit trees and palms. Moreover, the invention relates to forage plants (e.g. forage and pasture grasses, such as alfalfa, clover, ryegrass) and vegetable plants (e.g. tomato, lettuce, chicory) and ornamental plants (e.g. tulips, hyacinths). Sugar-storing and/or starch-storing plants are preferred. Sugar cane and sugar beet, and potato plants, maize, rice, wheat and tomato plants are particularly preferred.

A further subject of the invention is a method for the production of transgenic plant cells and transgenic plants which in comparison to non-transformed wildtype cells/non-transformed wildtype plants synthesize alternan. In this method the expression and/or the activity of proteins encoded by the nucleic acid molecules of the invention is increased in comparison to corresponding wild-type cells/wildtype plants which do not show any alternansucrase expression and/or activity. In particular, such a method comprises the expression of a nucleic acid molecule according to the invention in plant cells. The nucleic acid molecule according to the invention is preferably linked to a promoter ensuring expression in plant cells. In a particularly preferred embodiment the method comprises the introduction of a nucleic acid molecule according to the invention into a plant cell and regeneration of a plant from this cell.

Such an increase in expression may, e.g., be detected by Northern blot analysis. The increase in activity may be detected by testing protein extracts for their alternansucrase activity derived from plant cells. The enzymatic activity of an alternansucrase can be measured, for instance, as described in Lopez-Munguia et al. (Annals New York Academy of Sciences 613, (1990), 717-722) or as described in the examples of the present application.

The invention also relates to propagation material of the plants of the invention. The term “propagation material” comprises those components of the plant which are suitable to produce offspring vegetatively or generatively. Suitable means for vegetative propagation are for instance cuttings, callus cultures, rhizomes or tubers. Other propagation material includes for instance fruits, seeds, seedlings, protoplasts, cell cultures etc. The preferred propagation materials are tubers and seeds. The invention also relates to harvestable parts of the plants of the invention such as, for instance, fruits, seeds, tubers or rootstocks.

Another embodiment of the invention relates to methods for preparing alternan which comprise the step of extracting and isolating alternan from a plant of the invention.

The extraction and isolation of alternan from a plant of the invention may be carried out by standard methods, such as precipitation, extraction and chromatographic methods.

Moreover, the present invention relates to alternan obtainable from a plant of the invention or from propagation material of the invention.

Moreover, the present invention relates to a method for preparing alternan and/or fructose, wherein a host cell of the invention secretes an alternansucrase into a saccharose-containing culture medium and alternan and/or fructose is/are isolated from the culture medium.

A preferred embodiment of the method of the invention uses an alternansucrase recombinantly produced and secreted by the host cell into the culture medium, thus avoiding the necessity of breaking up the cells and purifying the protein further, because the secreted protein can be obtained from the supernatant. The residual components of the culture medium can be removed by methods usual in processing technology, such as dialysis, reverse osmosis, chromatographic methods, etc. The same applies to the concentration of the protein secreted into the culture medium. The secretion of proteins by microorganisms is normally mediated by N-terminal signal peptides (signal sequence, leader-peptide, transit peptide). Proteins possessing this signal sequence are able to penetrate the cell membrane of the microorganism. A secretion of proteins can be achieved by adding the DNA sequence encoding this signal peptide to the corresponding region encoding the alternansucrase.

The natural signal peptide of the expressed alternansucrase is preferred, that of the alternansucrase from Leuconostoc mesenteroides NRRL B 1355 (see the first approximately 25 to 45 N-terminal amino acid residues of Seq. ID No. 2) is particularly preferred.

The signal peptide of α-CGTase from Klebsiella oxytoca M5A1 (Fiedler et al., J. Mol. Biol. 256 (1996), 279-291) or a signal peptide as encoded by the nucleotides 11529-11618 of the sequence available under the GenBank accession number X86014 is most preferred.

The preparation of alternan and/or fructose requires neither activated glucose derivatives nor co-factors, as are necessary in most synthesis reactions for polysaccharides occurring within the cells. Hence, alternansucrase-secreting microorganisms can be cultured in saccharose-containing medium, the secreted alternansucrase leading to a synthesis of alternan and fructose in the culture medium.

Contrary to host cells from Leuconostoc mesenteroides, which secrete alternansucrase by nature, the host cells used according to the invention have the advantage that they do not secrete proteins possessing adverse polysaccharide-synthesizing side reactions, such as dextransucrase, with the result that outside the host cell, apart from alternan, no other polysaccharides can be formed which, as a rule, can be separated from alternan only by costly and time-consuming procedures. Moreover, the host cells according to a preferred embodiment of the invention do not have any adverse polysaccharide-degrading side activities, which would otherwise lead to losses in the yield of the alternan produced.

The method of the invention yields fructose apart from alternan. Fructose can be used for the inexpensive isolation of so-called “high-fructose-containing syrups” (HFCS). Conventional methods for preparing fructose on the one hand provide for the enzymatic break down of saccharose by means of an invertase or for the break down of starch into glucose units, mostly brought about by acid hydrolysis, and for subsequent enzymatic conversion of the glucose into fructose by glucose isomerases. However, both methods lead to mixtures of glucose and fructose. The two components must subsequently be separated from each other by chromatographic methods.

The separation of the two reaction products of the method of the invention, or the separation of the reaction products from the substrate saccharose can be achieved for example with the use of membranes permitting the penetration of fructose, but not the penetration of saccharose and/or alternans. If continuous removal of fructose via such a membrane is provided for, a more or less complete conversion of saccharose occurs.

The isolation of alternan and fructose can be carried out by standard methods or can be carried out as for instance described in the working examples.

According to one embodiment of the method, the host cells originate from microorganisms, preferably from Escherichia coli.

In another embodiment, the method of the invention works with fungal host cells, in particular cells of yeasts, such as Saccharomyces cerevisiae. Yeast cells producing alternan in saccharose-containing medium because of the enzymatic activity of an alternansucrase, cannot be readily used, as yeasts secrete an invertase which breaks down the extracellular saccharose. The yeasts take up the resulting hexoses via a hexose transporter. However, one yeast strain has been described (Riesmeier et al. EMBO J. 11 (1992), 4705-4713) which carries a defective suc2 gene, and therefore cannot secrete invertase. Moreover, these yeast cells do not contain a transportation system able to import saccharose into the cells. If such a strain is so modified by means of the nucleic acid molecules of the invention that it secretes an alternansucrase into the culture medium, then fructose and alternan will be synthesized in saccharose-containing medium. The resulting fructose can subsequently be taken up by the yeast cells.

In another preferred embodiment of this method the host cell of the invention is present in an immobilized form.

As a rule, host cells are immobilized by inclusion of the cells in a suitable material, such as alginate, polyacrylamide, gelatin, cellulose or chitosan. However, adsorption or covalent binding of the cells to a carrier material is also possible (Brodelius and Mosbach, Methods in Enzymology Vol. 135 (1987), 222-230) An advantage of the immobilization of cells is that it allows substantially higher cell densities to be achieved than does culturing in liquid culture. This results in a higher productivity. Moreover, the costs for agitation and aeration of the culture decrease as do the costs for measures to maintain sterility. Another important aspect is the possibility of a continuous alternan production with the result that unproductive phases regularly occurring in fermentation processes can be avoided or at least greatly reduced.

Another embodiment of the invention relates to a method for preparing alternan and/or fructose, wherein

-   a) a saccharose-containing solution is contacted with a protein of     the invention under conditions permitting the conversion of     saccharose into alternan and/or fructose; and -   b) alternan and/or fructose is/are isolated from the solution.

In this embodiment, the invention thus relates to a method for preparing alternan and/or fructose in vitro by means of a cell-free enzyme preparation. In this case, microorganisms which for instance secrete alternansucrase are cultured up to the stationary phase in a saccharose-free medium permitting the formation of alternansucrase protein. After removing the cells from the culture medium by centrifugation, the secreted enzyme can be recovered from the supernatant. The enzyme can subsequently be added to saccharose-containing solutions in order to synthesize alternan and/or fructose. Compared to the above-described synthesis of alternan in a system not freed from cells, this method offers the advantage that the reaction conditions can be controlled better and the reaction products are substantially purer and easier to purify. The purification of the protein can be carried out as already described above.

A preferred embodiment of the method of the invention uses a purified alternansucrase. Purified alternansucrase is understood to mean an enzyme which is largely free from cell components of the cells in which the protein is synthesized and shows no contamination with proteins possessing polysaccharide-synthesizing activities (e.g. dextransucrases) or degrading activities, and/or no contamination with (polysaccharide) acceptors. The term “purified alternansucrase” preferably means an alternansucrase possessing a degree of purity of at least 70%, preferably at least 85%, and particularly preferably at least 95%.

The use of a purified protein for preparing alternan and/or fructose offers various advantages. Compared to methods working with partially purified protein extracts, the reaction medium of the method of the invention does not contain any residues of the production strain (microorganism) which is used for the purification of the protein or for its preparation by genetic engineering.

Moreover, the use of the purified protein is advantageous for food and pharmaceutical industry applications. Thanks to the fact that the reaction medium is defined in its composition and freed from all unnecessary components, the product is likewise more precisely defined in respect of its components. In consequence of this, the procedure for obtaining food and pharmaceutical industry approval of these products produced by genetic engineering requires substantially less documentation, especially since these products should not show any traces of a transgenic microorganism.

Moreover, contrary to the so far described in vitro methods in cell free systems using partially purified alternansucrase preparations, the method of the invention using a purified alternansucrase has the advantage that it allows highly pure alternan to be prepared without the occurrence of dextransucrase and dextran contaminations, because of the high purity of the protein of the invention. Moreover, the method of the invention permits the production of alternan in high yields, without losses caused for instance by adverse side reactions of a dextransucrase, which would convert part of the substrate saccharose into undesired dextran, the separation of which from alternan would only be possible using time-consuming and expensive methods.

The method of the invention produces fructose in addition to alternan. The fructose can be used for the inexpensive recovery of so-called “high-fructose-containing syrups” (HFCS). The method of the invention yields products of high purity, because of the use of a purified alternansucrase. Hence, compared to conventional methods for preparing HFCS from maize starch, which comprise costly process steps for removing the buffer salts by ion exchange, (Crabb and Mitchinson, TIBTECH 15 (1997), 349-352) the method of the invention does not require an expensive purification of the fructose.

Another preferred embodiment of the method of the invention uses a recombinantly prepared alternansucrase.

According to another preferred embodiment, the enzyme possessing the enzymatic activity of an alternansucrase is immobilized on a carrier material. Immobilization of the alternansucrase offers the advantage that the enzyme being the catalyst of the synthesis reaction can be easily recovered from the reaction mixture and reused several times. As the purification of enzymes is normally costly and time consuming, immobilization and reutilization of the enzymes allow for a substantial cost saving. Another advantage is the degree of purity of the reaction products not containing any residual protein.

There are many carrier materials available for the immobilization of proteins, and coupling to the carrier material can be made via covalent or non-covalent bonds (for an overview see: Methods in Enzymology 135, 136, 137). Widely used carrier materials include for instance agarose, alginate, cellulose, polyacrylamide, silica or nylon.

According to another embodiment of the invention, the alternansucrase (immobilized on a carrier material) is present between two membranes, one of which allows fructose, but not saccharose and alternan to penetrate, the other one of which allows saccharose, but not alternan to penetrate. The supply with substrate occurs through the membrane which allows saccharose to penetrate it. The synthesized alternan remains in the space between the two membranes and the released fructose can be continuously removed from the reaction equilibrium via the membrane which only allows fructose to penetrate it. Such an arrangement permits an efficient separation of the reaction products, and thus the production of pure fructose.

Moreover, the separation of fructose by ion exchange chromatography has been described (“Starch Hydrolysis Products, Worldwide Technology, Production, and Application”, Edited by F. W. Schenck, R. E. Hebeda, (1992), VCH Publishers, Inc., New York).

Thus, the use of alternansucrases for preparing pure fructose on the one hand involves the advantage that the relatively inexpensive substrate saccharose can be used as the starting material, and on the other hand the fructose can be isolated easily from the reaction mixture without additional enzymatic conversions or chromatographic methods.

Moreover, the invention relates to methods for preparing alternan and/or fructose, wherein

-   a) a saccharose-containing solution is contacted with a protein of     the invention and acceptor molecules under conditions permitting the     conversion of saccharose to alternan and/or fructose; and -   b) alternan and/or fructose is/are isolated from the solution.

Within the framework of the present invention an acceptor molecule is understood to mean a molecule at which an alternansucrase is able to catalyze a chain-extending reaction. The acceptor which can be added to the reaction mixture at the beginning of the reaction is preferably a carbohydrate or a carbohydrate derivative. The use of external acceptors leads to the production of low molecular products which are to be designated alternan in the context of the present invention. The carbohydrate acceptor is preferably an oligo or polysaccharide, in particular a branched polysaccharide, such as dextrin, glycogen or amylopectin, preferably a linear polysaccharide, and particularly preferably a saccharide selected from the group consisting of maltose, isomaltose, isomaltotriose and methyl-α-D-glucan. If an extension of the alternan chain at these acceptors occurs, then products are formed which have a higher molecular weight than the educt. Where maltose, isomaltose, isomaltotriose and methyl-α-D-glucan are used, one obtains products which have a lower molecular weight than the alternan that can be prepared in the absence of external carbohydrate acceptors.

The size of the molecular weight of the oligoalternans prepared depends on the saccharose/acceptor ratio used. For instance the degree of polymerization of the products increases as the saccharose/isomaltose ratio increases. Moreover, the saccharose/acceptor ratio has an influence on the oligoalternan yield. For instance, the oligoalternan yield increases as the saccharose/isomaltose ratio decreases.

The hitherto described methods for producing oligoalternan with the use of alternansucrases which the authors claim have been purified (Pelenc et al., Sciences Des Aliments 11 (1991), 465-476) only yielded product mixtures of oligoalternan and oligodextran, in the presence of the carbohydrate acceptor maltose. In this case, the synthesis of oligodextran is presumably attributable to dextransucrase-contaminations of the alternansucrase preparation. Compared to this method, the method of the invention offers the advantage that the use of recombinantly produced alternansucrase protein not containing any dextransucrase contaminants permits the preparation of oligoalternan without the simultaneous formation of oligodextran. Thus, the method of the invention makes it possible to provide oligoalternan, without requiring additional costly purification steps for separating oligodextran.

According to another preferred embodiment, the enzyme possessing the enzymatic activity of an alternansucrase is immobilized on a carrier material.

According to another preferred embodiment of the method of the invention, a recombinantly produced alternansucrase is used.

Moreover, the present invention relates to end products containing alternan. In this context, end products are understood to mean cosmetic products, preferably food products, fodder and particularly preferably pharmaceutical products.

Finally, the present invention relates to a method for preparing the afore-mentioned products comprising one of the above-described alternan manufacturing methods of the invention and the formulation of the thus obtained alternan in a form which is suitable for one of the afore-mentioned applications of the corresponding product.

These and other embodiments are disclosed and obvious to a skilled person and embraced by the description and the examples of the present invention. Additional literature regarding one of the above-mentioned methods, means and applications, which can be used within the meaning of the present invention, can be obtained from the state of the art, for instance from public libraries for instance by the use of electronic means. This purpose can be served inter alia by public databases, such as the “Medline”, which are accessible via internet. Other databases and addresses are known to a skilled person and can be obtained from the internet. An overview of sources and information regarding patents and patent application in biotechnology is contained in Berks, TIBTECH 12 (1994), 352-364.

DESCRIPTION OF THE FIGURES

FIG. 1:

Linear map of the entire sequence region which was cloned after the screening of a genomic library of Leuconostoc mesenteroides NRRL B 1355 by the corresponding overlapping fragments of the clones AS-19B1, AS-19B2, AS-28B and AS-29Ba.

FIG. 2: Plasmid map pAlsu-pSK

FIG. 3: HPLC chromatogram: Preparation of oligoalternan in the presence of maltose (Example 2).

FIG. 4: Plasmid map pAlsu-pET24a

FIG. 5:

SDS PAGE with subsequent assay of sucrase activity (see Example 6)

The following protein extracts are used

1+2) E. coli BL21 (DE3) containing pAlsu-pET24a-3 3+4) E. coli BL21 (DE3) containing pAlsu-pET24a-7 5+6) E. coli BL21 (DE3) containing pAlsu-pET24a-21 7+8) E. coli BL21 (DE3) containing pET24a 1, 3, 5, 7) culture before induction with IPTG 2, 4, 6, 8) culture at the end of culturing

FIG. 6: HPLC chromatogram of dextran T10

FIG. 7: HPLC chromatogram of dextran T10 after dextranase digestion

FIG. 8: HPLC chromatogram of oligoalternan

FIG. 9: HPLC chromatogram of oligoalternan, after dextranase digestion.

FIG. 10: Map of the expression cassette including the polylinker of the plasmid pBinAR-N.

FIG. 11: Plasmid map pat-Alsu-Hyg.

FIG. 12: Plasmid map fnr-Alsu-Hyg.

EXAMPLES

Vectors used in the examples:

1. pBinAR-N

By the use of standard methods (Sambrook et al., Molecular cloning: A laboratory manual, 2nd issue; Cold Spring Harbor Laboratory Press, NY, USA (1989)) we introduced a different polylinker (see FIG. 10) between the 35S Promoter and the OCS-Terminator into the plasmid pBinAR (Höfgen und Willmitzer, Plant Science 66 (1990), 221-230). The resulting plasmid was called pBinAR-N.

2. pBinAR-Hyci-N

Via standard methods (Sambrook et al., Molecular cloning: A laboratory manual, 2nd issue; Cold Spring Harbor Laboratory Press, NY, USA (1989)) we isolated an EcoRI/HinDIII-fragment from pBinAR-N containing the 35S Promoter, the polylinker and the OCS-Terminator. This fragment was then ligated into the same restriction sites of plasmid pBIB-Hyg (Becker, Nucleic Acids Research 18 (1990), 203). The resulting plasmid was called pBinAR-Hyg-N.

3. pBinAR-pat-Hvg

By using the oligonucleotides Sp-pat-5′ and Sp-pat-3′ (s. SEQ ID Nos. 48 and SEQ ID No. 49) we amplified DNA molecules coding for the leader peptide of the patatin protein from potato (see SEQ ID No. 50, which differs from the sequence used by Sonnewald et al. Plant J. 1 (1991), 95-106) via a PCR approach using plasmid pgT5 (Rosahl et al., Mol. Gen. Genet. 203 (1986), 214-220; Sonnewald et al., Plant J. 1 (1991), 95-106) as a template. The resulting PCR products were cut by the restriction enzymes XbaI and SalI and then ligated into the plasmid pBinAR-Hyg-N which was linearized before by using the restriction enzymes SpeI and SalI. The resulting plasmid was called pBinAR-pat-Hyg.

PCR Conditions:

Buffer and polymerase from Boehringer Mannheim (Pwo Polymerase No. 1644947) DNA 0.2 ng 10 × Buffer + MgSO₄ 5 μl dNTPs (je 10 mM) 1 μl Primer Sp-pat-5′ 120 nM Primer Sp-pat-3′ 120 nM Pwo Polymerase 1.0 units distilled water ad 50 μl

Reaction Conditions:

Step 1 95° C. 2:30 min Step 2 95° C. 0:30 min Step 3 64° C. 0:30 min Step 4 72° C. 0:30 min (plus 1 sec per cycle) Step 5 72° C. 5:00 min.

The steps 2 to 4 were repeated 35 times in a cyclical manner.

4. pBinAR-FNR-Hyg

By using the oligonucleotides Sp-fnr-5′ and Sp-fnr-3 (see SEQ ID No. 51 and 52) we amplified DNA molecules coding for the transit peptide of the FNR protein from spinach via a PCR approach using plasmid p6SocFNR-15 (Jansen et al., Current Genetics 13, (1988), 517-522) as a template. The resulting PCR products were cut by XbaI and SalI and then cloned into the SpeI/SalI-opened pBinAR-Hyg-N. Resulting plasmid was called pBinAR-fnr-Hyg.

PCR Conditions:

Buffer and polymerase from Gibco BRL (Platinum Taq DNA Polymerase High Fidelity No. 1304-011) DNA 0.2 ng 10 × Buffer 5 μl MgSO4 2.0 μl dNTPs (per 10 mM) 1 μl Primer Sp-fnr-5′ 150 nM Primer Sp-fnr-3′ 150 nM Taq Platinum Hifi Polymerase 1.5 units distilled water ad 50 μl

Reaction Conditions:

Step 1 95° C. 2:30 min Step 2 95° C. 0:30 min Step 3 58° C. 0:30 min Step 4 68° C. 0:20 min (plus 1 sec per cycle) Step 5 68° C. 3:00 min

The steps 2 to 4 were repeated 35 times in a cyclical manner.

Example 1 Cloning of Alternansucrase from Leuconostoc mesenteroides NRRL-B1355 Isolation and Sequencing of Alternansucrase

The strain Leuconostoc mesenteroides NRRL-B1355 was cultured in 1 l of Lactobacilli MRS Broth (Difco) complemented with 5% saccharose at 28° C. for two days. After the culture was subjected to centrifugation at 20,000×g for 30 minutes, the supernatant was admixed with the same volume of 10% trichloro acetic acid and stirred at 4° C. for 16 hours. This solution was then subjected to centrifugation at 10,000×g for 30 minutes. The thus obtained precipitate was dissolved in 4.5 ml of 40 mM Tris-HCl, pH 8.8, and subsequently neutralized with (about 0.5 ml) 2 M Tris-base. This protein solution was given to the company Toplab Gesellschaft für angewandte Biotechnologie mbH, Martinsried, Germany, for protein sequencing. At this company, the protein solution was electrophoretically separated in SDS polyacrylamide gel, the gel was stained with Coomassie Blue and the staining was subsequently removed by 10% acetic acid. For the enzymatic digestion of the protein, the protein bands were cut from the gel, pressed through a sieve and fragmented (pores 30 μm×100 μm). The crushed gel was then washed with half concentrated incubation buffer (12.5 mM Tris, 0.5 mM EDTA pH 8.5) for 2 minutes. Subsequently, it was subjected to centrifugation, the buffer was removed and the gel was dried in the “Speedvac” for one hour (about 5% residual water, rubber-like). Subsequently, a solution of endoproteinase LysC in 400 μl 12.5 mM Tris/HCl, pH 8.5 (enzyme: protein=1: 10) and 0.1% of laurylmaltosite was prepared. 200 μl of this solution were added to the sample and incubated in the heat block shaker at 37° C. overnight. In order to elute the peptide fragments, a one hour incubation with 1% TFA was carried out, twice, followed by centrifugation, and subsequently by elution with 10% formic acid, 20% isopropanol, 60% acetonitrile for 3 hours. The peptide fragments obtained were then separated from each other by HPLC (column Superspher 60 RP select B (Merck, Darmstadt) 2 mm×125 mm; buffer A 0.1% trifluoro acetic acid, buffer B: 0.085% TFA in acetonitrile; flow rate: 0.2 ml/min; gradient: 5-60% in 60 min; detection at 206 nm. The peptide fragments obtained were then sequenced in an automatic sequencer Procise 492 (Applied Biosystems, PE); the procedure being the stepwise Edman degradation in a modification according to Hunkapiller (Hunkapiller et al., Meth. Enzymol. 91 (1983), 399-413).

Six different peptide sequences (see Seq. ID Nos. 5 to 9, Seq. ID No. 21) were identified which were designated lysC-66, lysC-67, lysC-82, lysC-83, lysC-88 and “N-terminus”.

Preparation of a Genomic DNA Library from Leuconostoc mesenteroides NRRL B1355

Leuconostoc mesenteroides NRRL-B1355 (purchased from ATCC) was cultured in 100 ml YT medium (Sambrook et al, loc. cit.) additionally containing 2% (w/v) of glucose and 50 mM sodium phosphate buffer pH 7.0, at 28° C. for 36 hours. After harvesting the cells by centrifugation, genomic DNA was isolated according to Ausubel et al. (Current Protocols in Molecular Biology, Volume 1, Greene and John Wiley & Sons (1994), USA).

100 μg of genomic DNA from Leuconostoc mesenteroides NRRL-B1355 were partially digested with 0.001 units of the restriction enzyme Sau3A for 30 minutes, subsequently extracted with phenol:chloroform:isoamyl alcohol (25:24:1) and precipitated with ethanol. 2.5 μg of the partially digested DNA obtained from Leuconostoc mesenteroides NRRL-B1355 were ligated with T4 DNA ligase in 1 μg of the BamHI-cut and dephosphorylated vector pBKCMVBamHI (Stratagene) under the conditions indicated by the manufacturer (Stratagene, pBK phagemid vectors instruction manual & T4 DNA ligase ligation kit). 2 μl of the ligation mixture were packaged with Gigapack III Gold (Stratagene) according to the instructions of the manufacturer and stored after the amount of phage content had been determined.

Preparation of the Probe for Isolating the Alternansucrase Gene

From the peptide sequences lysC-66 (Seq. ID No. 5), lysC-67 (Seq. ID No. 6), lysC-82 (Seq. ID No. 7), lysC-83 (Seq. ID No. 8) and lysC-88 (Seq. ID No. 9) obtained after tryptic digestion of the purified alternansucrase protein (see above) the peptides lysC-82 and lysC-83, after having undergone reverse translation, were selected for the synthesis of degenerated oligonucleotides (Seq. ID No. 10, Seq. ID No. 11). Said oligonucleotides served as primers in a PCR reaction on genomic DNA of NRRL-B1355. All positions within oligonucleotides depicted as N were replaced by inosin in the primer synthesis.

PCR Reaction Conditions

The reaction mixture was prepared with the buffers supplied for Taq polymerase (Company GibcoBRL).

Reaction Mixture:

Taq Polymerase (Gibco) DNA 100 ng (genomic NRRL-B1355) DNTPs 2.5 mM for each nucleotide primer 10 μl of a solution containing 0.2 μMol 10 fold buffer 5 μl magnesium chloride 2 mM polymerase 1 unit water ad 50 μl Step 1 95° C. 3′ Step 2 95° C. 1′ Step 3 58° C. 2′ Step 4 72° C. 2′ Step 5 72° C. 10′ 

40 repetitions of steps 2 to 4

An 837 bp fragment (Seq. ID No. 12) resulting from this PCR reaction, the ends of which were blunted with T4 DNA polymerase, was cloned into the SmaI-cut pBlueSkript vector (Stratagene). The resulting plasmid was designated pAlsu-PCR-lysc82/83. After sequencing of the insert and computer-aided translation into the corresponding protein sequences, a data base comparison was carried out in the Swiss Prot data base. This comparison showed homologies to known glycosyl transferases (P49331, P11001, P68987, P13470, P27470, P29336).

About 5,000 phages of the genomic DNA library of Leuconostoc mesenteroides NRRL-B1355 were plated out using the bacterial strains and nutrient solutions indicated by the manufacturer (Stratagene), and after incubation at 37° C. for 12 hours were transferred to nitrocellulose filters. This was followed by denaturation of the phages by immersion of the nitrocellulose filters in 1.5 m sodium chloride, 0.5 M caustic soda solution for 2 minutes and neutralization of the filters by immersion in 1.5 M sodium chloride, 0.5 M Tris-HCl, pH 8.0 for 5 minutes. After rinsing the filters in 0.2 M Tris-HCl, 2×SSC, the phage DNA was bound to the membranes by UV cross link (Stratalinker of the company Stratagene, 120,000 μJ for 30 seconds). The filters were incubated in a prehybridization solution (5×SSC, 0.5% BSA, 5×Denhardt, 1% SDS, 40 mM sodium phosphate buffer, pH 7.2, 100 mg/l herring sperm-DNA, 25% formamide) at 42° C. for 6 hours. 30 ng of the isolated insert from the plasmid pAlsu-PCR-lysc82/83 were radioactivley labeled by means of a multiprime kit (Boehringer Mannheim) using α-³²P dCTP (ICN Biomedicals). This radioactive probe was added to the prehybridization mixture and the filters were incubated in this hybridization mixture at 42° C. overnight. After removal of the hybridization mixture the filters were washed three times in a washing solution (0.1×SSC, 0.5% SDS) at 55° C. for 15 minutes. An X-ray film (Kodak) was then placed on the filter for 18 hours. Phage colonies, producing hybridization signals, were identified, isolated, resuspended in SM medium and then again plated out in a dissolution such that they could be recognized as single plaques. After these phages were transferred to nitrocellulose filters and subjected to further treatment and hybridization under conditions as described above, hybridizing phages were obtained as individual isolates by means of the radioactive gene probe used. After in vivo excision of the isolated phages in accordance with the manufacturer's instructions (Stratagene) the clones AS-19B1 and AS-19B2 could be isolated as plasmids. After complete sequencing of both clones (Agowa) (Seq. ID No. 13, Seq. ID No. 14) both sequences showed an 1008 bp overlap. The joining of Seq. ID No. 13 with Seq. No. 14 followed by computer aided translation of all possible reading frames allowed a continuous reading frame, starting with the codon ATG (corresponding to the bases 678 to 680 in Seq. ID No. 1), to be identified. As no stop codon could be found in this composed reading frame, additional clones were isolated in order to obtain the complete coding sequence of alternansucrase.

Therefore, about 5,000 phages of the genomic DNA library of L. mesenteroides NRRL-B1355 were again examined for hybridization by means of a clone AS-19B2 subfragment radioactively labeled using the multiprime kit (Boehringer Mannheim), as described above. The hybridization probe was prepared with the use of the HindIII (restriction site in the insert of AS-19B2)/SalI (cuts the pBKCMV phagemid vector in the polylinker)-fragment from AS-19B2. Said fragment contains 372 bases of the 3′ end of the sequences encoding the above-described reading frame. The screening of the phage library, singling out, and transformation of the phages into plasmids was carried out under the above-described conditions. After complete sequence analysis of the thus isolated clones AS-28B (see Seq. ID No. 15) and AS-29Ba (Seq. ID No. 16) it was possible to identify an overlap of 960 identical bases (corresponding to bases 4863 to 5823 in Seq. ID No. 1) between clones AS-19B2 (Seq. ID No. 14) and AS-28B and an overlap of 567 identical bases (corresponding to bases 5256 to 5823 in Seq. ID No. 1) between clones AS-19B2 and AS-29Ba (Seq. ID No. 16). Clones AS-28B and AS-29Ba have 1523 identical bases (corresponding to bases 5256 to 6779 in Seq. ID No. 1). After computer-aided joining of clones AS-19B1, AS-19-B2 and AS-28B a continuous reading frame starting with codon ATG (bases 678 to 680 on the complete sequence) appeared. This reading frame also does not contain a stop codon. After the joining of clones AS-19B1, AS-19B2, AS-28B and AS-29Ba it was possible to identify a reading frame starting with the codon “ATG” (corresponding to bases 678 to 680 in Seq. ID No. 1) and ending with “TAA” (corresponding to bases 6849 to 6851 in Seq. ID No. 1) encoding 2057 amino acids. In addition to the coding region, the entire isolated and identified DNA sequence of the composed clones (Seq. ID Nos. 13-16) contains 677 bases in the 5′ region and 2469 bases in the 3′ region which represent sequences not encoding alternansucrase (see FIG. 1).

Example 2 Construction of Plasmid pAlsu-pSK for the Transformation of E. coli and Test of the Protein Extracts for Enzymatic Activity

Plasmids AS-19B1, AS-19B2, AS-28B and AS-29Ba (see Example 1) were joined in the following manner: A NotI-(restriction site in the polylinker of vector pBK CMV, company Novagen)/ClaI-fragment of clone AS-19B1 was inserted into the vector pBluescript SK (company Stratagene) at the same restriction sites (=first cloning step). Consecutive insertion of the ClaI/XhoI fragment from AS-19B2, XhoI/Mlul fragment from AS-28B and Mlul/BsaBI (BsaBI-cut fragment cloned into the blunted ApaI restriction site of the vector) fragment of AS-28B into the clone obtained from the first cloning step produced plasmid pAlsu-pSK (see FIG. 2). This plasmid contains the complete coding sequence of the alternansucrase from Leuconostoc mesenteroides NRRL-B1355 as well as non-coding sequences of 677 bp (promoter region) in the 5′ region and 539 bp in the 3′ region (Seq. ID No. 17).

Plasmid pAlsu-pSK was then transformed in E. coli (DH5α company Fermentas). The bacteria were then cultured at 27° C. for two days in 50 ml “Terrific broth” (the composition of which is described in Sambrook et al., Molecular Cloning, A Laboratory Manual, 2^(nd) edition (1989) Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y. (supplemented with 0.5% glucose) or in a fermentation medium having the following composition: KH₂PO₄ 1.5 g/l, (NH₄)₂SO₄ 5.0 g/l, NaCl 0.5 g/l, Na-citrate 1.0 g/l, Fe²⁺SO₄×7H₂O, 0.075 g/l, yeast extract 0.5 g/l, tryptone 1.0 g/l, glucose 15.0 g/l, MgSO₄×7, H₂O0.3 g/l, CaCl₂×2H₂O 0.014 g/l, mineral salts 10 ml/l, H₃BO₃ 2.5 g/l, CoCl₂×6H₂O0.7 g/l, CuSO₄×5H₂O 0.25 g/l, MnCl₂×4H₂O, 1.6 g/l, ZnSO₄×7H₂O 0.3 g/l, Na₂MoO₄×2H₂O 0.15 g/l, vitamin B1 (thiamine) 0.005 g/l.

All cultures contained 100 mg/l ampicillin. The cells were then harvested by centrifugation, resuspended in 2 ml 50 mM Na-phosphate buffer pH 7.2 and crushed by a French Press. Subsequently, they were again subjected to centrifugation to remove solid particles of the crushed cells, and the supernatant (hereinafter referred to as (protein) extract) was used after sterilfiltration (Sterivex GV 0.2 μm, millipore) for further analyses.

In Vitro Preparation of Alternan by Means of Protein Extracts

For the in vitro preparation of alternan, 200 μl each of the extracts obtained were examined in 2 ml each of 100 mM Na-citrate buffer pH 6.5 and 20% (w/v) saccharose for activity in the presence and absence of 100 μl of 10 mM maltose. The reaction mixture was incubated at 37° C. for 24 hours. In the subsequent precipitation with the same volume of ethanol in the absence of maltose no precipitable polymer was found. In the batch containing maltose, HPLC chromatography (Dionex PA-100 column, running buffer 150 mM NaOH, elution buffer 150 mM NaOH+3 M sodium acetate buffer gradient) showed the formation of oligomers (see FIG. 3).

Activity Gel

20 ml each of the individual protein extracts were applied to a 6% SDS-PAA gel and separated at a current strength of 20 mA per gel. (Before application to the gels, the extracts were not incubated at 95° C.). Subsequently, the extracts were examined for sucrase activity according to the method of Miller and Robyt (Analytical Biochemistry 156 (1986), 357-363).

The control (dextransucrase NRRL-B-512F, see Example 3 for its preparation) showed polymerizing activity. The protein extracts of the above-described E. Coli cells containing the plasmid pAlsu-pSK, did not show any polymer-forming activity.

Example 3 Cloning and Expression of Dextransucrase from Leuconostoc mesenteroides NRRL-B512F Isolation of Genomic DNA

Leuconostoc mesenteroides NRRL-B512F (obtained from ATCC) was cultured at 28° C. for 48 hours in YT-medium (Sambrook et al., Molecular Cloning: A Laboratory Course Manual, 2^(nd) edition (1989), Cold Spring Harbor Press, New York) additionally containing 1% of saccharose and 50 mM sodium phosphate buffer pH 7.0. After harvesting the cells by centrifugation, genomic DNA was isolated according to Ausubel et al. (Current Protocols in Molecular Biology, Volume 1, Greene and John Wiley & Sons (1994), USA).

PCR Amplification of the Dextransucrase Gene and Cloning in pET24a

For the recombinant expression of dextransucrase in E. Coli, the gene encoding dextransucrase was cloned in the expression vector pET24a (Novagen) after PCR amplification. For this purpose, an Eagl restriction site was introduced at the 5′ end of the sequences encoding the dextransucrase and an XhoI restriction site at the 3′ end, together with the PCR primers used (5′b512-1: 5′-ACTgCggCCgCATgCCATTTACAgAAAAAg-3′; Seq. ID No. 3 and 3′b512: 5′-ACTgCTCgAgTTATgCTgACACAgCATTTC-3′; Seq. ID No. 4) derived from the sequence of WO 89/12386. Subsequent cloning into the corresponding restriction sites of the polylinker of pET24a was carried out. The resulting plasmid was designated UL5-20.

PCR Reaction Conditions

Buffer and polymerase of the company Gibco BRL were used.

DNA: 100 ng (genomic NRRL-B512F) 10 fold buffer 5 μl MgCl2 4 mM 5′ primer 50 ng 3′ primer 50 ng dNTP 1 mM of each nucleotide Pfu polymerase 0.5 units water ad 50 μl step 1 95° C. 4 minutes step 2 95° C. 1 minute  step 3 55° C. 1 minute  step 4 72° C. 5 minutes step 5 72° C. 10 minutes 

40 repetitions were made between steps 2 and 4.

Preparation of Recombinant Dextransucrase

BL21(DE3) E. Coli cells containing the plasmid UL5-20 were cultured in YT medium (see above) at 37° C. up to an OD 600=0.8. Subsequently, the cells were subjected to induction with 0.2 mM IPTG and cultured anew at 18° C. for 24 hours. After harvesting the cells by centrifugation and resuspending them in sodium phosphate buffer, pH 5.2, the cells were crushed in a French Press. The solution obtained was freed from insoluble components by centrifugation and the supernatant containing dextransucrase and referred to hereinafter as the extract was obtained.

Example 4 PCR Amplification of the Coding Region of Alternansucrase and Cloning in pET24a

The coding region of alternansucrase was amplified in a PCR reaction (see the reaction conditions below) with genomic DNA from the Leuconostoc mesenteroides strain NRRL-B1355 as a template. A Nhel restriction site was introduced at the 5′ end by means of primers A1-4 (Seq. ID No. 18), and a SalI-restriction site at the 3′ end by means of primer A1-5 (Seq. ID No. 19). A fragment of about 6200 bp was isolated.

A1-4: 5′-GGG CCC GCT AGC ATG AAA CAA CAA GAA ACA GT A1-5: 5′-CCC GGG GTC GAC CTT TGT CGA ATC CTT CCC

Reaction Conditions of the PCR (Kit of the Company Gibco BRL):

DNA 1 μl 10 × buffer 5 μl 10 mM per dNTP 2 μl 50 mM MgSO₄ 2 μl primer per 1 μl Platinum DNA polymerase 0.2 μl distilled water 37.8 μl step 1 95° C., 2 minutes step 2 95° C., 20 seconds step 3 47° C., 20 seconds step 4 68° C., 7 minutes (prolonged by 3 seconds per cycle) step 5 68° C., 15 minutes

Steps 2 to 4 were repeated 35 times altogether before step 5 was carried out.

The PCR fragment obtained was purified according to standard methods, treated with the restriction endonucleases Nhel and SalI, ligated into vector pET24a (of the company Novagen) which had likewise been cut with these enzymes, and the ligation product was transformed into E. Coli. After preparation of the plasmid and restriction digestion, three positive clones were selected. They were designated pAlsu-pET24a-3, pAlsu-pET24a-7 and pAlsu-pET24a-21 (see FIG. 4), respectively. All contained the sequence indicated in Seq. ID No. 20 as an insertion.

Example 5 Expression of the Recombinant Alternansucrase in E. coli in Shake Flask Cultures and in the Fermenter Shake Flask Culture

Plasmids pAlsu-pET24a-3, pAlsu-pET24a-7, pAlsu-pET24a-21 and pET24a were transformed into E. coli BL21 (DE3), of the company Novagen, and after initial culturing at 37° C. for 3 hours in 3 ml YT medium (Sambrook et al., Molecular Cloning, A Laboratory Manual, 2^(nd) edition (1989), Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y.) they were each cultured in shake flasks in 2 replicas in 50 ml Davis minimal medium (DIFCO Manual, Dehydrated Culture Media and Reagents for Microbiology, 10^(th) edition, Detroit Mich., USA (1984)) containing 0.2% glucose instead of dextrose as a carbon source at 37° C. until an OD₆₀₀ of about 0.8 was reached. After centrifugation and resuspension, one of the two replica cultures was cultured in Davis Minimal Medium (DMA) containing 1% lactose as the carbon source and inductor at 27° C. for another 16 hours. The cells of the individual cultures were harvested after centrifugation, resuspended in 50 mM sodium acetate buffer pH 5.3, and a protein extract was prepared as described in Example 2.

Fermenter

Clone pAlsu-pET24a-21 transformed in E. coli BL21 (DE3) was cultured in a 2 l fermenter (Biostad B; B.Braun, Melsungen) under the following conditions:

Medium:

Fermentation medium: KH₂PO₄ 1.5 g/l, (NH₄)₂SO₄ 5.0 g/l, NaCl 0.5 g/l, Na-citrate 1.0 g/l, Fe²⁺SO₄×7H₂O 0.075 g/l, yeast extract 0.5 g/l, tryptone 1.0 g/l, glucose 15.0 g/l, MgSO₄×7H₂O 0.3 g/l, CaCl₂×2H₂O 0.014 g/l, mineral salts 10 ml/l, H₃BO₃ 2.5 g/l, CoCl₂×6H₂O 0.7 g/l, CuSO₄×5H₂O 0.25 g/l, MnCl₂×4H₂O 1.6 g/l, ZnSO₄×7H₂O 0.3 g/l, Na₂MoO₄×2H₂O 0.15 g/l, vitamin B1 (thiamine) 0.005 g/l.

Carbon source: Glucose (1.5% (w/v)) is present in the medium, 70% (w/v) glucose solution is added.

Automatic pH control by ammonia and phosphoric acid at pH 7.0+/−0.1. A 20% concentration of pO₂ is adjusted in the medium via control by the stirrer.

Conditions:

-   1.5 l of fermentation medium were inoculated with 50 ml of the     preculture. The cells were first cultured at 37° C. until the     glucose present was consumed. They were then cultured at the same     temperature at a feeding rate of 9 g of glucose×l⁻¹×h⁻¹ until an     OD₆₀₀=40 was reached. At this time, the temperature of the culture     broth was lowered to 20° C. and the amount of glucose addition was     lowered to 2 g×l⁻²×h⁻¹. At a culture temperature of 20° C., the     culture was subjected to induction with 0.2 mM IPTG     (isopropyl-B-D-thiogalactopyranoside (Sigma)). After culturing at     20° C. for another 18 hours, the cells were harvested by     centrifugation, resuspended in 50 mM sodium phosphate buffer pH 5.3     and an extract was prepared as described in Example 2.

Example 6 SDS PAGE Assay of the Activity of the Recombinant Alternansucrase, Periodic Acid Oxidation and Staining according to Schiff

Protein extracts were prepared from E. coli shake flask cultures (strain BL21 (DE3)), containing the plasmids pAlsu-pET24a-3, pAlsu-pET24a-7, pAlsu-pET24a-21 and pET24a (control), respectively. Two different extracts were each prepared from the cells transformed with the different extracts, one of said extracts being prepared before induction with IPTG and the other one being prepared after induction with IPTG at the end of culturing. The activity of these extracts of shake flask cultures (see Example 5) was detected by SDS PAGE separation of the proteins, followed by SDS removal by washing with 50 mM sodium acetate buffer pH 5.3 and incubation of the gels in 50 mM sodium acetate pH 5.3, 5% (w/v) saccharose at 37° C. for 16 hours, followed by periodic acid oxidation of the polymer formed and staining by means of acidic Schiff reagent (Miller and Robyt, Analytical Biochemistry 156, (1986), 357-363).

FIG. 5 shows that sucrase activity has not been found for either one of the extracts (preparation of the extract before and after IPTG-induction) containing the cloning vector pET24a. In the case of strains which had been transformed with the plasmids pAlsu-pET24a-3, pAlsu-pET24a-7 and pAlsu-pET24a-21, respectively, all protein extracts showed sucrase activity at the end of the induction phase (concentrated in one band).

Before induction with IPTG such activity bands were not found.

As the polymer formed in the gel can be stained according to the above-described methods by acidic Schiff reagent, it can be assumed not to be composed of pure α-1,3-linked units which would not lead to any staining.

As the gene contained in vectors pAlsu-pET24a-3, pAlsu-pET24a-7 and pAlsu-pET24a-21, respectively, was isolated from the Leuconostoc mesenteroides strain NRRL-B1355 which expresses at least one dextran sucrase apart from alternansucrase, it was not possible to determine unambiguously with this staining method whether the nucleic acid sequence contained in the plasmid actually encodes an alternansucrase. Dextrans and alternans can both be detected by this method because both polymers contain α-1,6 linkages.

Example 7 Tests for the Enzymatic Activity of Recombinantly Prepared Alternansucrases after Heat Treatment and for the Specificity of Alternansucrase

In order to prove polymerization activities, extracts from shake flask cultures were used (see Example 5). 100 μl of extract were each added to 2 ml reaction buffer (50 mM sodium acetate pH 5.3, 20% saccharose) and incubated at 37° C. for 24 hours. For comparison, an extract inactivated by a 10 minute treatment at 95° C., and an extract from E. coli BL21 (DE3) containing vector pET24a were used. Polymer formation was only found in the batch that had not been inactivated, while the batch treated at 95° C. for 10 minutes and the batch with the extract from BL21 (DE3) containing pET24a did not show any polymer formation. After addition of the same volume of absolute ethanol to all batches, polymers could only be precipitated from the batch which had not been inactivated. This finding is a clear indication of the activity of alternansucrase, because the dextransucrase present in NRRL B-1355 is inactivated by a treatment at 45° C. for 30 minutes, while alternansucrase remains active under these conditions (Lopez-Munguia et al., Enzyme Microb. Technol. 15 (1993), 77-85). The enzymatic assay by a coupled enzymatic test of the glucose and fructose released and of the saccharose still contained in the reaction mixture after 24 hours, respectively, revealed that fructose was only present in the extract that was not inactivated.

For carrying out the enzymatic test either purified protein or crude protein extract is added in different dilutions to 1 ml batches containing 5% saccharose and 50 mM acetate, pH 5.5 and subjected to incubation at 37° C. After 5 minutes, 10 minutes, 15 minutes, 20 minutes, 25 minutes and 30 minutes, 10 μl each are removed from these batches and the enzymatic activity of alternansucrase is terminated by immediate heating to 95° C. Subsequently, in the coupled photometric test, the portions of fructose and glucose released by alternansucrase and the portion of used-up saccharose, respectively, are determined. For this purpose, 1 μl to 10 μl of the inactivated sample are placed into 1 ml of 50 mM imidazole buffer, pH 6.9, 2 mM MgCl₂, 1 mM ATP, 0.4 mM NAD and 0.5 U/ml hexokinase. After sequential addition of about 1 u of glucose-6-phosphate dehydrogenase (from Leuconostoc mesenteroides), about 1 u of phosphoglucose isomerase and about 5 u of invertase, the alteration of adsorption at 340 nm is measured. Subsequently, the amount of fructose and glucose released and used-up saccharose, respectively, is calculated according to the Lambert-Beer law.

In control batches (inactivation of the extract by treatment with 95° C. and extract from E. Coli containing pET24a) no significant release of fructose and no decrease of saccharose, respectively, was found in the reaction batch after 24 hours.

These results confirm that the specificity of the sucrase encoded by plasmids pAlsu-pET24a-3, pAlsu-pET24a-7 and pAlsu-pET24a-21, respectively, is that of a glucosyltransferase. The specificity of a fructosyl transferase, the presence of which has been described for some strains of the genus Leuconostoc is to be excluded, because otherwise glucose should have been found.

Example 8 Production of Alternan by Means of Alternansucrase Prepared in E. coli

100 ml of extract obtained by fermentation of E. Coli BL21(DE3) containing plasmid pAlsu-pET24a-3 (see Example 4) were added to 900 ml of reaction buffer (50 mM sodium acetate pH 5.3, 20% saccharose) and incubated at 37° C. for 24 hours. The addition of the same amount of absolute ethanol to the reaction mixture caused the alternan formed to precipitate. After the precipitate was washed twice with 50% ethanol, it was dried by lyophilization. The yield of dried polymer based on the amount of saccharose used in the reaction was 60%.

Example 9 HPLC Analysis of Alternan and Dextran After Dextranase Digestion

100 mg of the polymer prepared in Example 7 and 100 mg of dextran T10 (Pharmacia) were each dissolved in 1 ml of water. 40 μl each of these solutions were added to 700 μl reaction buffer (50 mM potassium phosphate pH 5.7, 8 units of dextranase, ICN Biomedicals Inc. No. 190097), and incubated at 37° C. for 16 hours. 50 μl of the polymer solutions not treated with dextranase (see FIG. 6) and 50 μl of the polymer solutions treated with dextranase (FIG. 7) were analyzed by HPLC (Dionex, column PA-100, NaOH/NaOH-NaAc gradient).

In the case of dextran T10 the cleavage of the polymer into different molecules of lower molecular weights can be clearly seen. The entire high molecular weight dextran is converted by dextranase into smaller units (mostly isomaltose). By contrast, in the case of alternan, short chained oligosaccharides only appear in small amounts after dextranase incubation. Most of the alternan is not digestible by dextranase. This finding suggests that the product prepared by recombinant alternansucrase is not dextran, but alternan which is known to be hardly accessible to enzymatic digestion by dextranase (Lopez-Mungia et al., Enzyme Microb. Technol. 15, (1993), 77-85).

Example 10 In Vitro Preparation of Alternan in the Absence of Dextranase

100 μl extract from shake flask cultures (see Example 5) were added to 2 ml of reaction buffer (50 mM sodium acetate, pH 5.3, 20% saccharose). 50 units of dextranase (Biomedicals Inc. No. 190097) were additionally added to another batch. Two corresponding batches which contained dextransucrase from Leuconostoc mesenteroides NRRL-B512F instead of the enzyme extract served as controls; one of these two batches had dextranase additionally admixed to it.

After precipitation with ethanol, the reaction batch with dextransucrase and dextranase did not show any polymer formation. All other batches were found to show polymer formation.

Example 11 In Vitro Preparation of Oligoalternan and HPLC Analysis

Oligoalternan was prepared as described in Example 2, with a protein extract in the presence of maltose and was subsequently detected (See FIG. 8) by HPLC-chromatography (see Example 2). For comparison, a portion of this batch was admixed with 50 units of dextranase (Biomedicals Inc. 190097) after preparation of oligoalternan and subsequently separation by HPLC chromatography was carried out as well (see FIG. 9). A comparison of the two chromatograms shows that not only the height of the two peaks which can be allocated to the oligoalternan α and β-anomer) (retention time between 15.87 and 16.61 minutes) but also the height of all the other peaks, the first signs of which are already visible without dextranase, remain unchanged. This finding suggests that recombinantly prepared alternansucrase allows oligoalternan to be prepared without the simultaneous production of oligodextran. Oligodextran would be liable to digestion by dextranase, which would have to show up in a decrease of the height of the peaks in the HPLC chromatogram, if oligodextran were present.

Example 12 Methylation Analysis of Alternan

In order to further analyze the alternan produced in vitro a methylation analysis was carried out:

Permethylation

The permethylation was performed as described by Ciucanu and Kerek (Carbohydr. Res. 131 (1984), 209-218) by using NaOH/MeI in DMSO or by using a modified method according to Hakomori (Journal of Biochemistry 55 (1964 FEB), 205-208) which relies on the use of freshly prepared Li-Dimsyl/MeI (Dimsyl=methylsulfinyl carbanion) in DMSO at room temperature.

All reactions are performed under a nitrogen atmosphere. The permethylation products are isolated by extracting the excess of methyliodide by the use of dichlormethan. DMSO and salts were washed out at the end.

Degradation into Partially Methylated Sorbitacetates (Methylation Analysis)

The permethylated glucans were hydrolyzed with 2N trifluorine acetic acid at 120° C. for 1-3 hours. After cooling the acid was removed by nitrogen. Then the resulting glucans were co-distilled with a small amount of toluene, afterwards reduced by NaBD₄ in 1 N ammonia and finally, acetylated by pyridine/acetanhydrid (3 h, 90° C.). The products were extracted by dichlormethan and washed with NaHCO₃. The products in the organic phase were analyzed by gas chromatography.

Analysis of the Acetylated Products

The acetylated products were analyzed by gas chromatography which was performed with a chromatograph manufactured by the Carlo-Erba company model GC 6000 Vega equipped with an on-column injector, a 25m CPSol8CB and a FID-detector. As a carrier gas hydrogen (80 kPa) was used.

The identification and integration of the peaks was performed as described by Sweet et al. (Carbohydr. Res. 40 (1975), 217).

Results

The following main components were identified by gas chromatography:

Sorbit acetylated in position Interpretation 1, 5 Terminal Glucopyranose 1, 3, 5 3-linked Glucopyranose 1, 5, 6 6-linked Glucopyranose 1, 3, 5, 6 3,6-linked Glucopyranose

Furthermore, small amounts (rel. amount 0.2-0.4 mol %) of the following components were also found: 1, 4, 5- and 1, 3, 4, 5-sorbit and another tetraacetyl component (1,5,x,y). It is supposed that these components are due to incomplete methylation.

The following amounts were found for the above mentioned components in different experiments which were performed by changing the length of hydrolysis (indicated in bold by the number of hours) (MA=methylation analysis1; MA-b=methylation analysis 2): Values in mol %

Ac in Pos MA (1 h) MA (2 h) MA (3 h) MA-b (2 h) 1, 5 10.49 10.56 9.17 12.71 1, 3, 5 31.69 34.70 32.95 23.12 1, 4, 5 0.70 0.30 0.36 0.33 1, 5, 6 47.02 44.17 47.23 54.62 1, 3, 4, 5 0.27 0.22 0.25 0.31 1, 5, x, y 0.19 0.32 0.36 0.24 1, 3, 5, 6 9.64 9.73 9.68 8.67

Example 13 Construction of an Expression Cassette for Plants: Vacuolar and Plastidic Expression of an Alternansucrase

By using plasmid Alsu-pET24a as a template and the PCR primers Al-5′-1.2 and Al-3′-2.2 (see SEQ ID NO 53 and 54) we amplified the coding region of alternansucrase from Leuconostoc mesenteroides which was then cut by the restriction enzymes SalI and PstI. Afterwards the resulting fragments were cloned into SalI and Sdal digested plasmids a) pBinAR-pat-Hyg and b) pBinAR-fnr-Hyg. The resulting plasmids were called a) pat-Alsu-Hyg (see FIG. 11) and b) fnr-Alsu-Hyg (see FIG. 12).

Note: The bacterial secretion signal peptide was removed from the cds by choice of the PCR primers.

PCR Conditions:

Buffer and polymerase from Boehringer Mannheim (Pwo Polymerase No. 1644947) DNA 0.5 ng 10 × Buffer + MgSO₄ 5 μl dNTPs (je 10 mM) 2 μl Primer Sp-AS-5′ 100 nM Primer Sp-AS-3′ 100 nM Pwo Polymerase 1.0 unit distilled water ad 50 μl

Reaction Conditions:

Step 1 95° C. 2:30 min Step 2 95° C. 0:30 min Step 3 47° C. 0:30 min Step 4 68° C. 7:00 min (plus 3 sec per cycle) Step 5 68° C. 15:00 min 

The steps 2 to 4 were repeated 35 times in a cyclical manner.

Example 14 Northern Blot Analysis for Expression of Alternansucrase in Transgenic Plants

Leaves or tubers from potato plants transformed via agrobacteria with plasmids pat-Alsu-Hyg and fnr-Alsu-Hyg, respectively, were pulverized in a mill, type MM 200, (Retsch GmbH & Co. KG, 42781 Haan, Germany) at 30 Hz for 50 sec. RNA was extracted according to Logemann et al. (Anal. Biochem. 163 (1987), 16-20). 50 μg RNA per sample were loaded on 1% agarose gels containing formaldehyde. After electrophoresis the RNA was transferred to nylon membranes (Hybond N, Amersham, UK) by the capillary transfer method (Sambrook et al., Molecular cloning: A laboratory manual, 2nd issue; Cold Spring Harbor Laboratory Press, NY, USA (1989)). Fixation of nucleic acids at the membrane was achieved by UV crosslinking (Stratalinker by Stratagene).

Membranes were prehybridized at 42° C. in hybridization buffer (25% (v/v) formamide, 250 mM sodium phosphate, pH 7.2, 250 mM sodiumchloride, 1 mM EDTA 7% (w/v) SDS, 25% (w/v) polyethyleneglycol 6000, 0.25 mg/ml sheared salmon sperm DNA) for 6 h. Afterwards hybridization was performed at 42° C. over night in hybridization buffer containing a radiolabelled probe in addition. The radioactive probe was prepared by using the Random Primed DNA Labelling Kit (Boehringer Mannheim, 1004760) and the approx. 4 kb KpnI/XhoI-fragment from plasmid pAlsu-pSK according to the manufacturer's manual. Membranes were washed at 50° C. once for 20 min in 3×SSC (Sambrook et al., Molecular cloning: A laboratory manual, 2nd issue; Cold Spring Harbor Laboratory Press, NY, USA (1989)) followed by washing once for 20 min in 0.5×SSC before exposing the membrane to an x-ray-film over night. 

1. A method for preparing alternan and/or fructose, wherein (a) a saccharose-containing solution is contacted with an alternansucrase protein under conditions permitting the conversion of saccharose into alternan and/or fructose, wherein the protein is encoded by a nucleic acid molecule selected from the group consisting of (i) a nucleic acid molecule encoding at least the mature form of a protein comprising the amino acid sequence indicated in SEQ ID NO: 2 or the amino acid sequence which is encoded by the insert of plasmid pAlsu-pSK within DSM 12666; (ii) a nucleic acid molecule comprising the nucleotide sequence indicated in SEQ ID NO: 1 or the nucleotide sequence within plasmid pAlsu-pSK within DSM 12666 or the corresponding ribonucleotide sequence; (iii) a nucleic acid molecule encoding a Leuconostoc mesenteroides protein comprising an amino acid sequence, the amino acid sequence of which has a homology of at least 90% to the amino acid sequence indicated in SEQ ID NO: 2; and (iv) an enzymatically active fragment of any of the above; and (b) alternan and/or fructose is/are isolated from the solution, wherein the protein of (a) shows no contamination with another Leuconostoc mesenteroides protein and catalyzes the conversion of sucrose to alternan.
 2. The method according to claim 1, wherein the protein is immobilized on a carrier material.
 3. A method for preparing alternan and/or fructose, wherein (a) a saccharose-containing solution is contacted with an alternansucrase protein and acceptor molecules of maltose, isomaltose, isomaltotriose or methyl-α-D-glucan under conditions permitting the conversion of saccharose into alternan and/or fructose, wherein the protein is encoded by a nucleic acid molecule selected from the group consisting of (i) a nucleic acid molecule encoding at least the mature form of a protein comprising the amino acid sequence indicated in SEQ ID NO: 2 or the amino acid sequence which is encoded by the insert of plasmid pAlsu-pSK within DSM 12666; (ii) a nucleic acid molecule comprising the nucleotide sequence indicated in SEQ ID NO: 1 or the nucleotide sequence within plasmid pAlsu-pSK within DSM 12666 or the corresponding ribonucleotide sequence; (iii) a nucleic acid molecule encoding a Leuconostoc mesenteroides protein comprising the amino acid sequence, the amino acid sequence of which has a homology of at least 90% to the amino acid sequence indicated in SEQ ID NO: 2; and (iv) an enzymatically active fragment of any of the above; and (b) alternan and/or fructose is/are isolated from the solution, wherein the protein of (a) shows no contamination with another Leuconostoc mesenteroides protein and catalyzes the conversion of sucrose to alternan.
 4. The method of claim 1 or 3, wherein said protein encoded by said nucleic acid molecule has a homology of at least 95% to the amino acid sequence indicated in SEQ ID NO:
 2. 5. The method of claim 1 or 3, wherein said protein encoded by said nucleic acid molecule is SEQ ID NO:
 2. 6. The method of claim 1, wherein said protein encoded by said nucleic acid molecule comprises residues 46 to 2057 of SEQ ID NO: 2 or residues 46 to 2057 of the amino acid sequence which is encoded by the DNA insert of plasmid pAlsu-pSK within DSM
 12666. 7. The method of claim 1, wherein said protein encoded by said nucleic acid molecule comprises residues 26 to 2057 of SEQ ID NO: 2 or residues 26 to 2057 of the amino acid sequence which is encoded by the DNA insert of plasmid pAlsu-pSK within DSM
 12666. 8. The method of claim 1, wherein said protein encoded by said nucleic acid molecule comprises residues 1 to 2057 of SEQ ID NO: 2 or residues 1 to 2057 of the amino acid sequence which is encoded by the DNA insert of plasmid pAlsu-pSK within DSM
 12666. 9. The method of claim 1 or 3, wherein said protein has at least 90% identity to the protein comprising (a) residues 46 to 2057 of SEQ ID NO: 2 or residues 46 to 2057 of the amino acid sequence which is encoded by the DNA insert of plasmid pAlsu-pSK within DSM 12666; (b) residues 26 to 2057 of SEQ ID NO: 2 or residues 26 to 2057 of the amino acid sequence which is encoded by the DNA insert of plasmid pAlsu-pSK within DSM 12666; or (c) residues 1 to 2057 of SEQ ID NO: 2 or residues 1 to 2057 of the amino acid sequence which is encoded by the DNA insert of plasmid pAlsu-pSK within DSM
 12666. 10. The method of claim 1 or 3, wherein said nucleic acid molecule comprises bases 813 to 6848 of SEQ ID NO: 1 or bases 813 to 6848 of the nucleotide sequence which is encoded by the DNA insert of plasmid pAlsu-pSK within DSM
 12666. 11. The method of claim 1 or 3, wherein said nucleic acid molecule comprises bases 753 to 6848 of SEQ ID NO: 1 or bases 753 to 6848 of the nucleotide sequence which is encoded by the DNA insert of plasmid pAlsu-pSK within DSM
 12666. 12. The method of claim 1 or 3, wherein said nucleic acid molecule comprises bases 678 to 6848 of SEQ ID NO: 1 or bases 678 to 6848 of the nucleotide sequence which is encoded by the DNA insert of plasmid pAlsu-pSK within DSM
 12666. 13. The method of claim 1 or 3, wherein said nucleic acid molecule has at least 90% identity to the nucleic acid molecule comprising (a) bases 813 to 6848 of SEQ ID NO: 1 or bases 813 to 6848 of the nucleotide sequence which is encoded by the DNA insert of plasmid pAlsu-pSK within DSM 12666; (b) bases 753 to 6848 of SEQ ID NO: 1 or bases 753 to 6848 of the nucleotide sequence which is encoded by the DNA insert of plasmid pAlsu-pSK within DSM 12666; or (c) bases 678 to 6848 of SEQ ID NO: 1 or bases 678 to 6848 of the nucleotide sequence which is encoded by the DNA insert of plasmid pAlsu-pSK within DSM
 12666. 14. (canceled)
 15. The method according to claim 3, wherein the protein is immobilized on a carrier.
 16. The method according to claim 1, wherein said conversion occurs in the presence of an oligosaccharide acceptor molecule at which an alternansucrase is able to catalyze a chain-extending reaction.
 17. The method of claim 1, wherein said conversion occurs in the absence of an oligosaccharide acceptor molecule at which an alternansucrase is able to catalyze a chain-extending reaction.
 18. The method of claim 2 or 15, wherein said carrier is selected from the group consisting of agarose, alginate, cellulose, polyacrylamide, silica and nylon.
 19. The method of claim 2 or 15, wherein said protein is immobilized via covalent or non-covalent coupling. 